Spin Relaxation

We report the first nuclear magnetic resonance ͑NMR͒ two-dimensional correlation T 1 -T 2 and T 2 -T 2 measurements of hydrating cement pastes. A small but distinct cross peak in the two-dimensional relaxation spectrum provides the first direct evidence of chemical exchange of water between gel and capillary pores occurring over the first 14 days of hydration. A correlation of features along the line T 1 =4T 2 provides strong supportive evidence for the surface diffusion model of 1 H nuclear spin relaxation in cements and for a multimodal discrete pore size distribution. Differences in detail of the results are reported for white cement paste and white cement paste with added silica fume. Both the method and the theory presented can be applied more widely to other high surface area materials with other reactive surface areas.

We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxations are too slow to affect trap lifetimes. However, 3 He-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag 3 He molecules, using ab initio calculation of Ag-He interaction potentials and spin interactions, quantum scattering theory, and direct Monte Carlo simulations to describe formation and spin relaxation in this system. The calculated rate of spin-change agrees quantitatively with experimental observations, providing indirect evidence for molecular formation in buffer-gas-cooled magnetic traps. The ability to cool and trap molecules holds great promise for new discoveries in chemistry and physics . Cooled and trapped molecules yield long interaction times, allowing for precision measurements of molecular structure and interactions, tests of fundamental physics , and applications in quantum information science . Chemistry shows fundamentally different behavior for cold molecules 8 , and can be highly controlled based on the kinetic energy, external fields 9 , and quantum states 10 of the reactants. Experiments with cold molecules have thus far involved two classes of molecules: Feshbach molecules and deeply bound ground-state molecules. Feshbach molecules are highly vibrationally excited molecules, bound near dissociation, which interact only weakly at long range, due to small dipole moments. They are created by binding pairs of ultracold atoms using laser light (photoassociation), ac magnetic fields (rf as-

We have measured the carrier spin dynamics in p-doped InAs/GaAs quantum dots by pumpprobe photo-induced circular dichroism and time-resolved photoluminescence experiments. We show that the hole spin dephasing is controlled by the hyperfine interaction between hole and nuclear spins. In the absence of external magnetic field, we find a characteristic hole spin dephasing time of 15 ns, in close agreement with our calculations based on dipole-dipole coupling between the hole and the quantum dot nuclei. Finally we demonstrate that a small external magnetic field, typically 10 mT instead of 200 mT for the case of electrons, quenches the hyperfine hole spin dephasing. An individual spin in a solid confined to a nanometer scale represents an ultimate quantum memory and is thus a potential candidate for the realization of spintronic and quantum information processing devices . Compared to bulk materials, the strong

The excitation of electron spin polarization and coherence by picosecond light pulses and their dynamics in a wide remotely doped quantum well are studied theoretically and experimentally. Assuming that all electrons in the quantum well are localized, the theory considers the resonant interaction of light pulses with the four-level system formed by the electron spins of the ground state and the hole spins of the trion excited state. The theory describes the effects of spontaneous emission, a transverse magnetic field and hole spin relaxation on the dynamics detected by the Kerr rotation of a probe pulse. Time resolved Kerr rotation experiments were carried out on a remotely doped 14 nm GaAs quantum well in the frequency range of optical transitions to the heavy hole ͑HH͒ trion and to the light-hole ͑LH͒ trion degenerate with the HH exciton. The experiments on the resonant excitation of the HH trion show a very slow heavy hole spin relaxation and, consequently, a weak electron spin polarization after the trion relaxation. In contrast, the resonant excitation of the LH trion/HH exciton results in a fast hole spin relaxation that increases electron spin polarization.

We have studied theoretically the electron spin relaxation in semiconductor quantum dots via interaction with nuclear spins. The relaxation is shown to be determined by three processes: (i) -the precession of the electron spin in the hyperfine field of the frozen fluctuation of the nuclear spins; (ii) -the precession of the nuclear spins in the hyperfine field of the electron; and (iii) -the precession of the nuclear spin in the dipole field of its nuclear neighbors. In external magnetic fields the relaxation of electron spins directed along the magnetic field is suppressed. Electron spins directed transverse to the magnetic field relax completely in a time on the order of the precession period of its spin in the field of the frozen fluctuation of the nuclear spins. Comparison with experiment shows that the hyperfine interaction with nuclei may be the dominant mechanism of electron spin relaxation in quantum dots.

We demonstrate high speed manipulation of a few-electron double quantum dot. In the one-electron regime, the double dot forms a charge qubit. Microwaves are used drive transitions between the (1,0) and (0,1) charge states of the double dot. A local quantum point contact charge detector measures the photon-induced change in occupancy of the charge states. Charge detection is used to measure T 1 ∼16 ns and also provides a lower bound estimate for T * 2 of 400 ps for the charge qubit. In the two-electron regime we use pulsed-gate techniques to measure the singlet-triplet relaxation time for nearly-degenerate spin states. These experiments demonstrate that the hyperfine interaction leads to fast spin relaxation at low magnetic fields. Finally, we discuss how two-electron spin states can be used to form a logical spin qubit.

Controlling decoherence is the most challenging task in realizing quantum information hardware 1-3 . Single electron spins in gallium arsenide are a leading candidate among solidstate implementations, however strong coupling to nuclear spins in the substrate hinders this approach 4-6 . To realize spin qubits in a nuclear-spin-free system, intensive studies based on group-IV semiconductor are being pursued. In this case, the challenge is primarily control of materials and interfaces, and device nanofabrication. We report important steps toward implementing spin qubits in a predominantly nuclear-spin-free system by demonstrating state preparation, pulsed gate control, and charge-sensing spin readout of confined hole spins in a one-dimensional Ge/Si nanowire. With fast gating, we measure T 1 spin relaxation times in coupled quantum dots approaching 1 ms, increasing with lower magnetic field, consistent with a spin-orbit mechanism that is usually masked by hyperfine contributions. Since Loss and DiVincenzo's proposal 1 , the promise of quantum dots for solid state quantum computation has been underscored by the successful initialization, manipulation and readout of electron spins in GaAs systems 5,7-9 . The electronic wave functions in these systems typically overlap with a large number of nuclear spins that are difficult to control and in most cases thermally randomized. The resulting intrinsic spin decoherence rates 4-6 have been successfully reduced by spin-echo techniques 6,10 but require complex gate sequences that complicate multiqubit operations 11 . The prospect of achieving long coherence times in group IV materials with

The spin of a confined electron, when oriented originally in some direction, will lose memory of that orientation after some time. Physical mechanisms leading to this relaxation of spin memory typically involve either coupling of the electron spin to its orbital motion or to nuclear spins. Relaxation of confined electron spin has been previously measured only for Zeeman or exchange split spin states, where spin-orbit effects dominate relaxation, while relaxation due to nuclei has been observed in optical spectroscopy studies. Using an ...

2 . Temperature dependent STS measurements showed Fano resonances resulting from the hybridization between local moments (V) and itinerant electrons (Fe) below 100 K, and the formation of Fano lattice implying collective spin excitations between local moments of V atoms below 50 K. QPI analysis showed replica bands and kink features in Fe-itinerant band, implying the existence of Bosonic modes between Sr 2 VO 3 layers and FeAs layers. Our results show the collective behaviors of itinerant electrons and local moments, and the possibility of local moments contributing to superconductivity.

Spin relaxation taking place during radiofrequency (RF) irradiation can be assessed by measuring the longitudinal and transverse rotating frame relaxation rate constants (R 1ρ and R 2ρ ). These relaxation parameters can be altered by utilizing different settings of the RF irradiation, thus providing a useful tool to generate contrast in MRI. In this work we investigate the dependencies of R 1ρ and R 2ρ due to dipolar interactions and anisochronous exchange (i.e., exchange between spins with different chemical shift δω≠0) on the properties of conventional spin-lock and adiabatic pulses, with particular emphasis on the latter ones which were not fully described previously. The results of simulations based on relaxation theory provide a foundation for formulating practical considerations for in vivo applications of rotating frame relaxation methods. Rotating frame relaxation measurements obtained from phantoms and from the human brain at 4T are presented to confirm the theoretical predictions.

We develop the microscopic theory of the extrinsic spin-Hall conductivity of a two-dimensional electron gas, including skew-scattering, side-jump, and Coulomb interaction effects. We find that while the spin-Hall conductivity connected with the side jump is independent of the strength of electron-electron interactions, the skew-scattering term is reduced by the spin-Coulomb drag, so the total spin current and the total spin-Hall conductivity are reduced for typical experimental mobilities. Further, we predict that in paramagnetic systems the spin-Coulomb drag reduces the spin accumulations in two different ways: ͑i͒ directly through the reduction of the skew-scattering contribution, and ͑ii͒ indirectly through the reduction of the spin diffusion length. Explicit expressions for the various contributions to the spin-Hall conductivity are obtained using an exactly solvable model of the skew scattering.

The possible quantum phase transition has been investigated in non-superconducting Ni-substituted La$_{2-x}$Sr$_x$Cu$_{1-y}$Ni$_y$O$_4$, in which the superconductivity observed in La$_{2-x}$Sr$_x$CuO$_4$ is suppressed by the Ni substitution without disturbing the ...

Photo-induced electron transfer (PET) between a poly(3-dodecylthiophene) (P3DDT) and maleic anhydride (MA) in their blend in liquid solution of tetrahydrofuran was observed by the time-resolved electron spin resonance (TRESR) under UV flash photolysis at 308 nm. The observed spectra were identified as free-radical signals of positive polarons on polymer chains and MA anion radicals. Their emissive chemical induced dynamic electron polarization (CIDEP) originated mainly from excited triplet states (triplet mechanism of CIDEP). Analysis of radical spectra integral intensity distribution shows at the influence of radical pair mechanism (RPM) of CIDEP with the positive sign of the exchange interaction constant (J N 0). The last is attributed to Coulombic interaction between geminately formed polarons and MA anion radicals. The spin relaxation times of radicals were determined by fitting the time evolution of the TRESR signal at nearresonance positions of the field using the Bloch equations and direct Fourier transform analysis.

We present measurements of magnetic field and frequency dependences of the low temperature (T = 1.8 K) AC-susceptibility, and temperature and field dependences of the longitudinal field positive muon spin relaxation (μSR) for LiY 1-x Ho x F 4 with x = 0.0017, 0.0085, 0.0408, and 0.0855. The fits of numerical simulations to the susceptibility data for the x = 0.0017, 0.0085 and 0.0408 show that Ho-Ho cross-relaxation processes become more important at higher concentrations, signaling the crossover from single-ion to correlated behavior. We simulate the muon spin depolarization using the parameters extracted from the susceptibility, and the simulations agree well with our data for samples with x = 0.0017 and 0.0085. The μSR data for samples with x = 0.0408 and 0.0855 at low temperatures (T < 10 K) cannot be described within a single-ion picture of magnetic field fluctuations and give evidence for additional mechanisms of depolarization due to Ho 3+ correlations. We also observe an unusual peak in the magnetic field dependence of the muon relaxation rate in the temperature interval 10 -20 K that we ascribe to a modification of the Ho 3+ fluctuation rate due to a field induced shift of the energy gap between the ground and the first excited doublet crystal field states relative to a peak in the phonon density of states centered near 63 cm -1 .

A study of the elastic exciton-exciton Coulomb scattering in a semiconductor quantum well is presented, including the interexciton exchange of carriers and the spin degrees of freedom. The theoretical results show that electron-electron and hole-hole exchanges are the dominant mechanisms of interaction, while the classical direct term is negligible. The density-dependent homogeneous linewidth is calculated within the Born approximation and good agreement with the existing experimental data is obtained. Owing to the interexciton exchange of carriers, collisions lead to spin relaxation as actually observed in recent time-resolved photoluminescence experiments. ͓S0163-1829͑98͒06736-8͔

Tellurium impurity centers in ZnSe were individually probed with time-resolved photoluminescence (PL) spectroscopy. Resolution-limited peaks with an ultra-low spatial density originate in the recombination of excitons deeply bound to isolated nearest-neighbor isoelectronic Te pairs (Te 2 ). This interpretation is confirmed by ab-initio calculations. The peaks reveal anti-bunched photon emission and a doublet structure polarized along ] 110 [ and ] 10 1 [ . We analyze the time-resolved PL decay to clarify the role of the dark states in the spin relaxation and radiative recombination of single fine-structure split excitons.

The typical demagnetization mechanism is applicable in magneto-optic recording systems on a nanoseconds timescale. To develop data processing speed in communication systems and the recording industry, much faster magnetization dynamics must occur on a timescale of femtosecond. Interest in this area has expanded rapidly since understanding the physics of ultrafast magnetization processes by experiment are an acutely challenging task. As a result, in order to explain the quenching of magnetization by laser heating in ferromagnetic alloys such as permalloy thin film with thickness varying from 1 nm to 5 nm, we executed a new theoretical study. In particular, we demonstrate that the magnetization decays in a timescale of picoseconds by the microscopic three-temperature model.

The ESR intensity, line shape, and longitudinal electron-spin relaxation in the polymer phase of the RbC 60 fulleride are investigated in the temperature range 4.2ϽTϽ300 K. Most attention is focused on the metalinsulator transition region ͑25-50 K͒. It is found that below 50 K the ESR line can be separated into two Lorentzian components ascribed to conduction electrons and some localized paramagnetic centers ͑with concentration of about 0.03 per formula unit͒ with allowance made for the relaxation bottleneck. The decrease of the conduction-electron susceptibility obeys an activation law with the characteristic energy ⌬/k B ϭ80 Ϯ10 K related to the opening of a gap 2⌬Ϸ100 cm Ϫ1 . The same quantity is found by analyzing both longitudinal and transverse relaxation caused by fluctuations of internal fields with correlation time c ϰ exp(2⌬/k B T). Below 25 K, the temperature dependencies of the linewidth and the relaxation times change abruptly, revealing the development of a new ordered state. The nature of this state is discussed. ͓S0163-1829͑97͒06135-3͔

Using original modulation technique, the longitudinal electron spin-relaxation time T 1 has been measured directly in the paramagnetic state of three La 1Ϫx Ca x MnO 3 samples (xϭ0.2, 0.25, and 0.33͒. Well above the phase-transition temperature T c , the longitudinal relaxation times are found to be equal to the transversal ones (T 2 ) as determined from the electron-paramagnetic-resonance ͑EPR͒ linewidth, whereas a steep slowing down of T 1 with a critical exponent ␣Ӎ0.5 was observed as T c was approached in all the materials. Various models are discussed, including extremely slow internal dynamics, formation of magnetic clusters, and inhomogeneous EPR broadening near T c .

Single crystals of diamagnetically diluted manganites, LaGa 1Ϫx Mn x O 3 (xϭ0 -0.2) have been investigated by means of magnetic-resonance techniques. Detailed study was performed of the NMR line shape, spin-spin and spin-lattice nuclear relaxation of 69 Ga and 71 Ga nuclei. The analysis of the temperature and concentration dependencies of the spin-relaxation rates was supported by additional measurements on 139 La nuclei and electron-spin resonance on Mn electron-spin system. The electrical quadrupole mechanism of nuclear-spin relaxation is shown to be dominant at low Mn concentration (xр0.005), whereas the magnetic dipolar interaction with the Mn electron spins is prevalent at xу0.02. The obtained data suggest the presence of thermally activated internal motion with the activation energy of about 50 meV that governs the Mn 3ϩ electron spinlattice relaxation and, via dipolar interaction, the nuclear-spin relaxation of the host nuclei. In the scenario suggested, this motion can be related to the hindered Jahn-Teller dynamics of the Mn 3ϩ ions linked to clusters or pinning defects.

The hysteresis cycles have been studied and hysteresis losses have been calculated by integrating the area of the hysteresis cycle on deuterated organic superconductor (κ-D8-Br) κ-(BEDT-TTF)2Cu[N(CN)2]Br. The measurements were obtained for different temperature values and as a function of the cooling rate through the order-disorder transformation near 80 K. Our results showed that the hysteresis cycles areas and hysteresis loss of these compounds depend strongly on the temperature and the cooling rate VC.

H. El Ouaddi1,*, A. Tirbiyine1, A. Taoufik1, Y. Ait Ahmed1, A. Hafid1, M. Mamor2, H. Chaib3, A. Nafidi3, S. Snoussi4 1E.M.S, Research Laboratory, Faculty of Science, University Ibn Zoh, Agadir, Morocco 2M.S.I.S.M, Research Laboratory, Polydisciplinary Faculty, University Cadi Ayyad, Safi, Morocco 3L.P.M.C.N.E.R, Research Laboratory, Faculty of Science, University Ibn Zoh, Agadir, Morocco 4Laboratoire de Physique des Solides, UMR 8502, Université Paris-Sud, Bat.510, 91405 Orsay, France *E-mail: hassanelouaddi@gmail.com

We report investigations of the low temperature dc susceptibility and the magnetization on the layered organic superconductor k-(BEDT-TTF)2Cu[N(CN)2]Br near 80 K and the effect of disorder on the superconducting transition temperature Tc. The shielding effect (S) and the critical current density Jc were studied (with H parallel to the c axis of the crystal). Jc can be estimated by analysis of magnetic hysteresis measurement using the Bean model. For each temperature value, we observed two regimes in the critical current density Jc(H). Our results show that the magnetic properties of these compounds depend strongly on the cooling rate. The structural transformation which occurs at the vicinity of 80 K very strongly influences the physics of vortex lattice and the associated magnetic behavior. #organic_superconductor #critical_current #shielding_effect #magnetic_susceptibility #vortex_pinning

Dynamics of spin-lattice relaxation of magnetic Mn ions in semimagnetic Cd 0.99 Mn 0.01 Te/Cd 0.76 Mg 0.24 Te quantum wells containing two-dimensional electron gas ͑2DEG͒ has been studied by means of an optical detection of injected nonequilibrium phonons. It is found that the spin-lattice relaxation rate is an increasing function of electron concentration and electron temperature. The 2DEG provides an effective channel for the energy transfer from magnetic ions to the lattice. A model accounting for the spin-flip transitions between Mn ions and free electrons describes well our experimental results in a qualitative manner.

Controlling decoherence is the most challenging task in realizing quantum information hardware 1-3 . Single electron spins in gallium arsenide are a leading candidate among solidstate implementations, however strong coupling to nuclear spins in the substrate hinders this approach 4-6 . To realize spin qubits in a nuclear-spin-free system, intensive studies based on group-IV semiconductor are being pursued. In this case, the challenge is primarily control of materials and interfaces, and device nanofabrication. We report important steps toward implementing spin qubits in a predominantly nuclear-spin-free system by demonstrating state preparation, pulsed gate control, and charge-sensing spin readout of confined hole spins in a one-dimensional Ge/Si nanowire. With fast gating, we measure T 1 spin relaxation times in coupled quantum dots approaching 1 ms, increasing with lower magnetic field, consistent with a spin-orbit mechanism that is usually masked by hyperfine contributions. Since Loss and DiVincenzo's proposal 1 , the promise of quantum dots for solid state quantum computation has been underscored by the successful initialization, manipulation and readout of electron spins in GaAs systems 5,7-9 . The electronic wave functions in these systems typically overlap with a large number of nuclear spins that are difficult to control and in most cases thermally randomized. The resulting intrinsic spin decoherence rates 4-6 have been successfully reduced by spin-echo techniques 6,10 but require complex gate sequences that complicate multiqubit operations 11 . The prospect of achieving long coherence times in group IV materials with

The diffusion of positive muons was studied in the normal and superconducting states of aluminum. Large differences were observed, indicating that the difFusion mechanism depends sensitively on the in- teraction between the muon and the conduction electrons. The samples were high-purity aluminum doped with controlled amounts of lithium. The lithium impurities act as traps for the muons, leading to a loss of muon polarization. However, the time and temperature dependencies of the muon depolariza- tion function 6 (t) could not be satisfactorily explained using a simple diffusion limited trapping model. In particular, in the superconducting state the measured G(t) requires a more complex model for a reasonable fit. These data can be described, in a first approximation, with two fractions, one correspond- ing to muons stopped in the defect potential created by the doping element and the other representing muons which diffuse freely without being trapped. In this model, which supports a microscopic theory by Kagan and Prokof'ev taking into account static as well as dynamic effects on the tunneling of positive particles in a conducting medium, a large fraction of the muons is practically immobile below 0.3 K in doped samples. The data can also be fitted by assuming that the spatial distribution of doping elements leads to a distribution in trapping times and that the fraction of diffusing muons is described by a so- called stretched exponential time dependence. The relative merits of these two types of interpretation are discussed.

Alkali vapor cells with antirelaxation coating (especially paraffin-coated cells) have been a central tool in optical pumping and atomic spectroscopy experiments for 50 years. We have discovered a dramatic change of the alkali vapor density in a paraffin-coated cell upon application of an electric field to the cell. A systematic experimental characterization of the phenomenon is carried out for electric fields ranging in strength from 0-8 kV/cm for paraffin-coated cells containing rubidium and cells containing cesium. The typical response of the vapor density to a rapid (duration < ∼ 100 ms) change in electric field of sufficient magnitude includes (a) a rapid (duration of < ∼ 100 ms) and significant increase in alkali vapor density followed by (b) a less rapid (duration of ∼ 1 s) and significant decrease in vapor density (below the equilibrium vapor density), and then (c) a slow (duration of ∼ 100 s) recovery of the vapor density to its equilibrium value. Measurements conducted after the alkali vapor density has returned to its equilibrium value indicate minimal change (at the level of < ∼ 10%) in the relaxation rate of atomic polarization. Experiments suggest that the phenomenon is related to an electric-field-induced modification of the paraffin coating.

The magnetoconductivity of Ga x In 1-x As/ InP quantum wires with widths in the range of 1220-250 nm was investigated. The finite zero-field spin splitting in our samples gives rise to spin relaxation and weak antilocalization in wide wires. In contrast, for the narrow wires, only weak-localization behavior is seen even though the zero-field spin splitting is independent of wire width. The observed renormalization of the spin-relaxation length due to purely geometrical effects can be described quantitatively using a model where the effect of spin precession is represented by spin-dependent pseudomagnetic fluxes, and by exact numerical transport calculations.

Quasi-one dimensional edge channels are formed at the boundary of a two-dimensional electron system subject to a strong perpendicular magnetic field. We consider the effect of Rashba spin-orbit coupling, induced by structural inversion asymmetry, on their electronic and transport properties. Both our analytical and numerical results show that spin-split quantum-Hall edge channels exhibit properties analogous to that of Rashba-split quantum wires. Suppressed backscattering and a long spin life time render these edge channels an ideal system for observing voltage-controlled spin precession. Based on the latter, we propose a magnet-less spin-dependent electron interferometer.

Spin-polarised atomic hydrogen is used as a gaseous polarised proton target in high energy and nuclear physics experiments operating with internal beams in storage rings. When such beams are intense and bunched, this type of target can be depolarised by a resonant interaction with the transient magnetic field generated by the beam bunches. This effect has been studied with the HERA positron beam in the HERMES experiment at DESY. Resonances have been observed and a simple analytic model has been used to explain their shape and position. Operating conditions for the experiment have been found where there is no significant target depolarisation due to this effect.

Motivated by recent experiments on molecular quantum dots we investigate the relaxation of pure spin states when coupled to metallic leads. Under suitable conditions these systems are well described by a ferromagnetic Kondo model. Using two recently developed theoretical approaches, the time-dependent numerical renormalization group and an extended flow equation method, we calculate the real-time evolution of a Kondo spin into its partially screened steady state. We obtain exact analytical results which agree well with numerical implementations of both methods. Analytical expressions for the steady state magnetization and the dependence of the long-time relaxation on microscopic parameters are established. We find the long-time relaxation process to be much faster in the regime of anisotropic Kondo couplings. The steady state magnetization is found to deviate significantly from its thermal equilibrium value.

The Langevin capture model is often used to describe barrierless reactive collisions. At very low temperatures, quantum effects may alter this simple capture image and dramatically affect the reaction probability. In this paper, we use the trajectory-ensemble reformulation of quantum mechanics, as recently proposed by one of the authors (Poirier) to compute adiabatic-channel capture probabilities and cross-sections for the highly exothermic reaction Li + CaH(v = 0, j = 0) → LiH + Ca, at low and ultra-low temperatures. Each captured quantum trajectory takes full account of tunneling and quantum reflection along the radial collision coordinate. Our approach is found to be very fast and accurate, down to extremely low temperatures. Moreover, it provides an intuitive and practical procedure for determining the capture distance (i.e., where the capture probability is evaluated), which would otherwise be arbitrary.

Order or disorder often exists in a uniform spin system consisting of one kind of magnetic ion. Nevertheless, they rarely coexist in normal conditions. Our thermodynamic and microscopic magnetic studies of Co 2 OH 3 Cl, a distorted tetrahedral lattice compound with uniform Co 2 spin, demonstrate that the spins located on one corner of the tetrahedron are periodically ordered, but those on the other three are disordered below a ferromagnetic transition at T C 10:5 K. The partial order resembles that of the fieldinduced ''kagome ´-ice'' state in spin ice pyrochlore compounds. Evidence suggests that a distortion in the tetrahedron is responsible for this partial ferromagnetic order in a zero field.

Magnetic semiconductors have aroused interest due to their various functionalities related to spintronic devices. Manganese (Mn) as a substitutional impurity in A3B5 semiconductors supplies not only holes, but also localized spins. The ejection of Mn atoms with an uncompensated magnetic moment leads to the appearance of ferromagnetic properties. The most suitable material characterized by long-term spin dynamics is n-type GaAs. In p-type GaAs, the spin relaxation time of electrons is generally much shorter. For purposes of this research, electron-spin relaxation times in 3D and 2D p-type GaAs were studied. Calculation of impurity densities and charge state of magnetic acceptors demonstrate the essential composition of the material. Comparison of theoretical and experimental data in optical-spin orientation of electrons reveal the longest spin relaxation time of 77 ns in 2D GaAs:Mn, less than twice the best time in the p-type 3D GaAs material.

To optimize simulations of CW EPR spectra for high-spin Fe(III) with zero-field splitting comparable to the EPR quantum, information is needed on the factors that contribute to the line shapes and line widths. Continuous wave electron paramagnetic resonance (EPR) spectra obtained for iron transferrin carbonate from 4 to 150 K and for iron transferrin oxalate from 4 to 100 K did not exhibit significant temperature dependence of the line shape, which suggested that the line shapes were not relaxation determined. To obtain direct information concerning the electron spin relaxation rates, electron spin echo and inversion recovery EPR were used to measure T 1 and T m for the high-spin Fe(III) in iron transferrin carbonate and iron transferrin oxalate between 5 and 20-30 K. For comparison with the data for the transferrin complexes, relaxation times were obtained for tris(oxalato)ferrate(III). The relaxation rates are similar for the three complexes and do not exhibit a strong dependence on position in the spectrum. Extrapolation of the observed temperature dependence of the relaxation rates to higher temperatures gives values consistent with the conclusion that the CW line shapes are not relaxation determined up to 150 K.

Van der Waals α-MoO3 samples offer a wide range of attractive catalytic, electronic, and optical properties. We present herein an emission Mössbauer spectroscopy (eMS) study of the electric-field gradient (EFG) anisotropy in crystalline free-standing α-MoO3 samples. Although α-MoO3 is a two-dimensional (2D) material, scanning electron microscopy shows that the crystals are 0.5–5-µm thick. The combination of X-ray diffraction and micro-Raman spectroscopy, performed after sample preparation, provided evidence of the phase purity and crystal quality of the samples. The eMS measurements were conducted following the implantation of 57Mn (t1/2 = 1.5 min), which decays to the 57Fe, 14.4 keV Mössbauer state. The eMS spectra of the samples are dominated by a paramagnetic doublet (D1) with an angular dependence, pointing to the Fe2+ probe ions being in a crystalline environment. It is attributed to an asymmetric EFG at the eMS probe site originating from strong in-plane covalent bonds and wea...

We examined the magnetic resonance properties of 12 paramagnetic piperidinyl nitroxyls in water and plasma solutions. Paramagnetic contributions to proton relaxation times were measured using 10.7 and 100 MHz spectrometers. Proton relaxation enhancement from nitroxyls increased with ascending molecular weight, in plasma solutions versus equimolar aqueous solutions, and with measurements at 10.7 MHz compared to 100 MHz. Relaxation rates were observed to approximately double at 10.7 MHz compared to 100 MHz and from water to plasma solutions. The data indicate that proton spin-lattice relaxation enhancement is magnetic field-dependent, and increases using nitroxyls of large molecular weight and with chemical substitutents that increase the microviscosity of solvent water molecules. The development of nitroxyls for diagnostic MRI will be aided by understanding these in vitro physical characteristics and trends.

Some features of the experimental data on nuclear spin relaxation time T, in the heavy-fermion superconducting state can be explained by taking into account the effect of the electron Zeeman energy. It is found that at intermediate temperatures the usual quasiparticle spin-flip scattering dominates, while at very low temperatures a new process, pair creation (annihilation), dominates and gives T; ' a T.

We study the dynamics of two ensembles of atoms (or equivalently, atomic clocks) coupled to a bad cavity and pumped incoherently by a Raman laser. Our main result is the nonequilibrium phase diagram for this experimental setup in terms of two parameters -detuning between the clocks and the repump rate. There are three main phases -trivial steady state (Phase I), where all atoms are maximally pumped, nontrivial steady state corresponding to monochromatic superradiance (Phase II), and amplitude-modulated superradiance (Phase III). Phases I and II are fixed points of the mean-field dynamics, while in most of Phase III stable attractors are limit cycles. Equations of motion possess an axial symmetry and a Z2 symmetry with respect to the interchange of the two clocks. Either one or both of these symmetries are spontaneously broken in various phases. The trivial steady state loses stability via a supercritical Hopf bifurcation bringing about a Z2-symmetric limit cycle. The nontrivial steady state goes through a subcritical Hopf bifurcation responsible for coexistence of monochromatic and amplitude-modulated superradiance. Using Floquet analysis, we show that the Z2-symmetric limit cycle eventually becomes unstable and gives rise to two Z2-asymmetric limit cycles via a supercritical pitchfork bifurcation. Each of the above attractors has its own unique fingerprint in the power spectrum of the light radiated from the cavity. In particular, limit cycles in Phase III emit frequency combs -series of equidistant peaks, where the symmetry of the frequency comb reflects the symmetry of the underlying limit cycle. For typical experimental parameters, the spacing between the peaks is several orders of magnitude smaller than the monochromatic superradiance frequency, making the lasing frequency highly tunable.

Magnetically induced spin relaxation in InAs QDs M. YASAR, I. KHAN, T. ALI, A. PETROU, SUNY at Buffalo, Buffalo NY, G. KIOSEOGLOU, C. LI, A. HANBICKI, B. JONKER, Naval Research Laboratory, Washington D.C., M. KORKUSINSKI, Institute for Microstructural Sciences NRC, Ottawa -The circular polarization P of light emitted by Fe/InAs QDs spin LEDs has been studied as function of magnetic field Band temperature T . P shows a pronounced decrease around B o = 5 T, in the form of a resonance with a full width of ≈ 0.75 T with the following characteristics: (i) The resonance strength is quite sensitive to the bias voltage V . At low V the resonance is strong, but, as V is increased, it becomes progressively weaker. (ii) The resonance is pronounced at T = 5 K but loses strength with increasing temperature, and disappears above 60 K. The decrease in P around B o is attributed to a spin relaxation mechanism that is induced by magnetic field. The sensitivity of the resonance to V suggests that the origin of the spin relaxation mechanism is connected to the spin-orbit interaction. By changing B we tune the energies of different electron states and thereby change the rate of spin relaxation in the system. We compare experimental results with calculations, in which many-body energies and wave functions are obtained using the effective-mass configurationinteraction approach; the spin-orbit interaction, is treated perturbatively. Work at SUNY was supported by ONR and NSF. M. Yasar SUNY at Buffalo, Buffalo NY

We have studied the domain wall dynamics in Joule-annealed amorphous glass-covered microwires with positive magnetostriction in the presence of an electric current, in order to evaluate the profile and shape of the moving domain wall. Such microwires are known to present magnetic bi-stability when axially magnetized. The single domain wall dynamics was evaluated under different conditions, under an axially applied stress and an electric current. We have observed the well known increasing of the domain wall damping with the applied stress due to the increase in the magnetoelastic anisotropy and, when the current is applied, depending on the current intensity and direction, a modification on the axial domain wall damping. When the orthogonal motion of the domain wall is considered, we have observed that the associated velocity present a smaller dependence on the applied current intensity. It was observed a modification on both the domain wall shape and length. In a general way, the domain wall evolves from a bell shape to a parabolic shape as the current intensity is increased. The results were explained in terms of the change in the magnetic energy promoted by the additional Oersted field.

We have studied the domain wall dynamics in Joule-annealed amorphous glass-covered microwires with positive magnetostriction in the presence of an electric current, in order to evaluate the profile and shape of the moving domain wall. Such microwires are known to present magnetic bi-stability when axially magnetized. The single domain wall dynamics was evaluated under different conditions, under an axially applied stress and an electric current. We have observed the well known increasing of the domain wall damping with the applied stress due to the increase in the magnetoelastic anisotropy and, when the current is applied, depending on the current intensity and direction, a modification on the axial domain wall damping. When the orthogonal motion of the domain wall is considered, we have observed that the associated velocity present a smaller dependence on the applied current intensity. It was observed a modification on both the domain wall shape and length. In a general way, the domain wall evolves from a bell shape to a parabolic shape as the current intensity is increased. The results were explained in terms of the change in the magnetic energy promoted by the additional Oersted field.

We present measurements of magnetic field and frequency dependences of the low temperature (T = 1.8 K) AC-susceptibility, and temperature and field dependences of the longitudinal field positive muon spin relaxation (μSR) for LiY 1-x Ho x F 4 with x = 0.0017, 0.0085, 0.0408, and 0.0855. The fits of numerical simulations to the susceptibility data for the x = 0.0017, 0.0085 and 0.0408 show that Ho-Ho cross-relaxation processes become more important at higher concentrations, signaling the crossover from single-ion to correlated behavior. We simulate the muon spin depolarization using the parameters extracted from the susceptibility, and the simulations agree well with our data for samples with x = 0.0017 and 0.0085. The μSR data for samples with x = 0.0408 and 0.0855 at low temperatures (T < 10 K) cannot be described within a single-ion picture of magnetic field fluctuations and give evidence for additional mechanisms of depolarization due to Ho 3+ correlations. We also observe an unusual peak in the magnetic field dependence of the muon relaxation rate in the temperature interval 10 -20 K that we ascribe to a modification of the Ho 3+ fluctuation rate due to a field induced shift of the energy gap between the ground and the first excited doublet crystal field states relative to a peak in the phonon density of states centered near 63 cm -1 .

We have studied polycrystalline Yb 4 LiGe 4 , a ternary variant of the R 5 T 4 family of layered compounds characterized by a very strong coupling between the magnetic and crystallographic degrees of freedom. The system is mixed valent, with non-magnetic Yb 2+ and magnetic Yb 3+ present, and is characterized by coexisting ferromagnetic and antiferromagnetic correlations. We present measurements of resistivity, AC-susceptibility, specific heat, and muon spin relaxation (μSR), below 1 K. The low temperature measurements suggest a transition to a mesoscopically inhomogeneous magnetically ordered state below 2 K characterized by fluctuations well below the ordering temperature. This unusual state is believed to result from the enhanced twodimensionality produced by Li substitution and frustration effects inherent in the Yb sub-lattice geometry. (T G = 240 K) into ferromagnetic clusters , and long-range antiferromagnetic order is observed for temperatures below the Néel temperature T N = 128 K. At lower temperatures a Martensiticlike structural transition facilitates a transition from AFM to FM order. While most of the R 5 T 4 materials exhibit either ferro-or antiferromagnetic order at temperatures ranging from tens to hundreds of degrees Kelvin, one member of the family, Yb 5 Ge 4 , has a reported onset of antiferromagnetic order at roughly 2 K which has only been observed via Mossbauer spectroscopy . The system is mixed-valent, with the Yb assuming the non-magnetic Yb 2+ and magnetic Yb 3+ valence states in comparable proportions. The existence of several competing interactions with comparable magnitude, as evidenced by the low transition temperature, is expected to result in unusual properties. In this work we study a derivative material, Yb 4 LiGe 4 . Detailed studies of the chemistry and structure of this material [9, 10] have shown that the Li substitutes for the Yb in lattice sites such that the layers containing Yb 3+ ions are separated by Li containing layers (see Fig. ), suggesting that the material will tend towards two-dimensional behavior. Measurements of specific heat, 7 Li spin-lattice relaxation, and magnetic susceptibility demonstrated that the system was characterized by coexisting ferromagnetic and antiferromagnetic correlations . In this work, we extend our studies to lower temperatures, and present measurements of resistivity, AC susceptibility, specific heat, and muon spin relaxation (μSR) to below T = 1 K. We find evidence for the onset of unusual magnetic ordering below 2 K. The persistence of magnetic fluctuations to 100 mK as determined by the μSR data imply that quantum fluctuations play an important role in this system. A detailed description of our fabrication and characterization of Yb 4 LiGe 4 and Yb 5 Ge 4 samples is presented elsewhere [10], and is generally consistent with the results reported by Xie et al. . We present an overview of the process here. The starting materials were ingots of ytterbium (99.99 wt%), lithium rods (99.4 wt%), and germanium pieces (99.9999 wt%). The elements in stoichiometric ratios were heated in closed tantalum tubes to roughly 1070 K using a high frequency (HF) furnace. Careful handling of the ytterbium and lithium was necessary in order to minimize impurities, and was carried out inside an argon glove-box system. The

The onset of correlation effects in the magnetic Ho$^{3+}$-subsystem in LiHo$_{x}$Y$_{1-x}$ F$_{4}$ single crystals is studied by comparing measurements and simulations of the field and frequency dependent magnetic AC susceptibility at 1.8 K and field and temperature ...

Temperature and magnetic field dependences of the 19 F nuclear spin-lattice relaxation in a single crystal of LiYF 4 doped with holmium are described by an approach based on a detailed consideration of the magnetic dipole-dipole interactions between nuclei and impurity paramagnetic ions and nuclear spin diffusion processes. The observed nonexponential long time recovery of the nuclear magnetization after saturation at intermediate temperatures is in agreement with predictions of the spin-diffusion theory in a case of the diffusion limited relaxation. At avoided level crossings in the spectrum of electron-nuclear states of Ho 3+ ions, rates of nuclear spin-lattice relaxation increase due to quasi-resonant energy exchange between nuclei and paramagnetic ions in contrast to the predominant role played by electronic cross-relaxation processes in the low-frequency ac-susceptibility.

Biomolecules undergo motions on the micro-to-millisecond timescale to adopt low-populated transient states that play important roles in folding, recognition, and catalysis. NMR techniques such as CPMG, CEST and R 1ρ are the most commonly used methods for characterizing such transitions at atomic resolution under solution conditions. CPMG and CEST are most effective at characterizing motions on the millisecond timescale. While some implementations of the R 1ρ experiment are more broadly sensitive to motions on the micro-to-millisecond timescale, they entail the use of selective irradiation schemes and inefficient 1D data acquisition methods. Here, we show that high power radio-frequency fields can be used in CEST experiments to extend sensitivity to faster motions on the micro-to-millisecond timescale. Given the ease of implementing high power fields in CEST, the work described here should make it easier to characterize micro-to-millisecond dynamics in biomolecules.

Thermodynamic propensities of biomolecules to adopt non-native conformations are crucial for understanding how they function, but prove difficult to measure experimentally. Combining optical melting experiments with chemical modifications and mutations, we developed delta-Melt for measuring the energetic penalties associated with nucleic acid conformational rearrangements and how they vary with sequence and physiological conditions. delta-Melt is fast, simple, cost effective, and can characterize conformational penalties inaccessible to conventional biophysical methods.