We report the experimental observation of quantized conductance by Rashba bands at the surface of... more We report the experimental observation of quantized conductance by Rashba bands at the surface of 50-nm Bi nanowires. With increasing magnetic fields along the wire axis, the wires exhibit a stepwise increase of conductance with as many as four distinct plateaus together with oscillatory thermopower. The observations can be accounted for by the increase of the number of propagating surface modes. The modes have very high mobility and are associated with Aharonov-Bohm interference around the perimeter consistently with theory of quasiballistic one-dimensional nanowires considering that Lorentz forces decouple the modes from scattering at the nanowire surface.
We report on the magnetic properties of Eu 2 Ir 2 O 7 upon the application of hydrostatic pressur... more We report on the magnetic properties of Eu 2 Ir 2 O 7 upon the application of hydrostatic pressure P by means of macroscopic and local-probe techniques. In contrast to previously reported resistivity measurements, our dc magnetization data unambiguously demonstrate a nonmonotonic dependence of T N , i.e., the critical transition temperature to the magnetic phase, on P . Strikingly, the recently calculated behavior for T N closely reproduces our results under the assumption that P lowers the U/W ratio (i.e., Coulomb repulsion energy over electronic bandwidth). Zero-field muon-spin spectroscopy measurements confirm that the Ir 4+ magnetic moment and/or the local magnetic configuration are only weakly perturbed by low P values. Accordingly, our current experimental findings strongly support the preservation of a 4-in/4-out ground state across the accessed region of the phase diagram.
We present resistivity, magnetization, and zero field muon spin relaxation (µSR) data for the pyr... more We present resistivity, magnetization, and zero field muon spin relaxation (µSR) data for the pyrochlore iridate materials Nd2-xCaxIr2O7 (x = 0, 0.06, and 0.10) and Sm2Ir2O7. While Nd2Ir2O7 (Nd227) is weakly conducting, Sm2Ir2O7 (Sm227) has a slowly diverging resistivity at low temperature. Nd227 and Sm227 exhibit magnetic anomalies at TM = 105K and 137K, respectively. However, zero-field µSR measurements show that long-range magnetic order of the Ir 4+ sublattice sets in at much lower temperatures (TLRO ∼ 8K for Nd227 and 70K for Sm227); both materials show heavily damped muon precession with a characteristic frequency near 9 MHz. The magnetic anomaly at TM in Nd227 is not significantly affected by the introduction of hole carriers by Ca-substitution in the conducting Nd2-xCaxIr2O7 samples, but the muon precession is fully suppressed for both.
We present measurements of magnetic field and frequency dependences of the low temperature (T = 1... more 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 ... more 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
We report a muon spin relaxation/rotation, bulk magnetization, neutron scattering, and transport ... more We report a muon spin relaxation/rotation, bulk magnetization, neutron scattering, and transport study of the electronic properties of the Nd 2 Ir 2 O 7 . We observe the onset of strongly hysteretic behavior in the temperature dependent magnetization below 120 K, and an abrupt increase in the temperature dependent resistivity below 8 K. Muon spin relaxation measurements show that the hysteretic magnetization is driven by a transition to a magnetically disordered state, and below 8 K a magnetically ordered ground state sets in, as evidenced by the onset of spontaneous muon precession. Our measurements point toward the absence of a true metal-to-insulator phase transition in this material and suggest that Nd 2 Ir 2 O 7 may lie within or on the metallic side of the boundary of the Dirac semimetal regime within its topological phase diagram.
The angular and temperature (10-250 K) variation of the Knight shift of single-crystalline U(Pt 0... more The angular and temperature (10-250 K) variation of the Knight shift of single-crystalline U(Pt 0.95 Pd 0.05 ) 3 has been measured in transverse field (B=0.6 T) µSR experiments. By analysing the temperature variation of the Knight shift with a modified Curie-Weiss expression the muon localization site in this hexagonal material is determined at (0,0,0).
The onset of correlation effects in the magnetic Ho$^{3+}$-subsystem in LiHo$_{x}$Y$_{1-x}$ F$_{4... more The onset of correlation effects in the magnetic Ho$^{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 ...
The realization of spin liquid states born from the near-critical regime of the triangular lattic... more The realization of spin liquid states born from the near-critical regime of the triangular lattice Hubbard model in inorganic materials remains a long-standing challenge, where weak spin-orbit coupling and other small perturbations often induce conventional spin freezing or order. Strong enough spin-orbit coupling, however, can renormalize the electronic wave function and induced anisotropic exchange interactions that promote magnetic frustration. Through the cooperative interplay of spin-orbit coupling and correlation effects, here we show that the triangular lattice magnet NaRuO2 hosts an inherently fluctuating magnetic ground state with thermodynamic properties suggestive of a crossover between dynamic ground states. Despite the presence of a charge gap, we find that low-temperature spin excitations generate a metal-like term in the specific heat and continuum excitations in neutron scattering, reminiscent of spin liquid states found in triangular lattice organic magnets. Further cooling reveals that these fluctuations crossover into a state whose dynamic spin autocorrelation function reflects persistent fluctuations within a highly disordered spin state. These findings instantiate NaRuO2 as a unique, Heisenberg-Kitaev cousin to organic, Heisenberg spin liquid compounds with a lowtemperature crossover in quantum disorder driven via the interplay between geometric frustration, extended hopping, and relativistic spin-orbit coupling.
Entangled spin states are created by implanting muons into single-crystal LiY 0.95 Ho 0.05 F 4 to... more Entangled spin states are created by implanting muons into single-crystal LiY 0.95 Ho 0.05 F 4 to form a cluster of correlated, dipole-coupled local magnetic moments. The resulting states have well-defined energy levels allowing experimental manipulation of the state populations by electromagnetic excitation. Experimental control of the evolution of the muon spin polarization is demonstrated through application of continuous, radio-frequency electromagnetic excitation fields. A semiclassical model of quantum, dipolecoupled spins interacting with a classical, oscillating magnetic field accounts for the muon spin evolution. On application of the excitation field, this model shows how changes in the state populations lead to the experimentally observed effects, thus enabling a spectroscopic probe of entangled spin states with muons.
The stretched diamond lattice material LiYbO 2 has recently been reported to exhibit two magnetic... more The stretched diamond lattice material LiYbO 2 has recently been reported to exhibit two magnetic transitions (T N1 = 1.1 K, T N2 = 0.45 K) via specific heat, magnetization, and neutron scattering measurements [Bordelon et al., Phys. Rev. B 103, 014420 (2021)]. Here we report complementary magnetic measurements down to T = 0.28 K via the local-probe technique of muon spin relaxation. While we observe a rapid increase in the zero-field muon depolarization rate at T N1 , for T < T N1 we do not observe the spontaneous muon precession which is typically associated with long-range magnetic ordering. The depolarization rate in the ordered state shows a surprising sensitivity to magnetic fields applied along the initial spin polarization direction. Using a simple one-dimensional model, we show that these results are consistent with the unusual random-phase bipartite incommensurate magnetic structure proposed by Bordelon et al. for the intermediate temperature range T N2 < T < T N1 . We also find evidence for magnetic fluctuations persisting to our lowest temperatures, but no obvious signature of the transition or spontaneous muon precession at and below T N2 , respectively.
Recent observations of novel spin-orbit coupled states have generated interest in 4 d /5 d transi... more Recent observations of novel spin-orbit coupled states have generated interest in 4 d /5 d transition metal systems. A prime example is the J eff = 1 2 state in iridate materials and α-RuCl 3 that drives Kitaev interactions. Here, by tuning the competition between spin-orbit interaction (λ SOC ) and trigonal crystal field (Δ T ), we restructure the spin-orbital wave functions into a previously unobserved μ = 1 2 state that drives Ising interactions. This is done via a topochemical reaction that converts Li 2 RhO 3 to Ag 3 LiRh 2 O 6 . Using perturbation theory, we present an explicit expression for the μ = 1 2 state in the limit Δ T ≫ λ SOC realized in Ag 3 LiRh 2 O 6 , different from the conventional J eff = 1 2 state in the limit λ SOC ≫ Δ T realized in Li 2 RhO 3 . The change of ground state is followed by a marked change of magnetism from a 6 K spin-glass in Li 2 RhO 3 to a 94 K antiferromagnet in Ag 3 LiRh 2 O 6 .
We report on the investigation of the magnetic superconductor EuFe 2 (As 0.7 P 0.3 ) 2 based on m... more We report on the investigation of the magnetic superconductor EuFe 2 (As 0.7 P 0.3 ) 2 based on muon-spin spectroscopy and ac magnetic susceptibility (χ) measurements. The dependence of the internal field at the muon site on temperature is indicative of a ferromagnetic ordering of Eu 2+ magnetic moments and only the conventional magnon scattering governs the longitudinal relaxation rate at low temperatures. At the same time, we observe a rich phenomenology for the imaginary component of the susceptibility χ by means of both standard ac susceptibility and a novel technique based on a microwave coplanar waveguide resonator. In particular, we detect activated trends for several features in χ over frequencies spanning ten orders of magnitude. We interpret our results in terms of the complex dynamics of vortices and antivortices influenced by the underlying structure of magnetic domains.
Chinese Academy of Sciences, Beijing 100190, P. R. China Laboratory for Muon Spin Spectroscopy, P... more Chinese Academy of Sciences, Beijing 100190, P. R. China Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, 5232 Villigen, Switzerland Institute for Quantum Electronics, ETH Zürich, 8093 Zürich, Switzerland SwissFEL, Paul Scherrer Institute, Villigen, Switzerland Department of Physics, Boston College, Chestnut Hill, MA 02467, USA University of Chinese Academy of Sciences, Beijing 100049, P. R. China CAS Center for Excellence in Topological Quantum Computation, University of Chinese
We have carried out muon spin relaxation and rotation measurements on the newly discovered kagome... more We have carried out muon spin relaxation and rotation measurements on the newly discovered kagome metal KV3Sb5, and find a local field dominated by weak magnetic disorder which we associate with the nuclear moments present, and a modest temperature dependence which tracks the bulk magnetic susceptibility. We find no evidence for the existence of V4+ local moments, suggesting that the physics underlying the recently reported giant unconventional anomalous Hall effect in this material warrants further studies.
We present an in-depth analysis of muon-spin spectroscopy measurements of Eu2Ir2O7 under the effe... more We present an in-depth analysis of muon-spin spectroscopy measurements of Eu2Ir2O7 under the effect of the Eu1-xBix isovalent and diamagnetic substitution as well as of external pressure. Our results evidence an anomalously slow increase of the magnetic volume fraction upon decreasing temperature only for stoichiometric Eu2Ir2O7, pointing towards highly unconventional properties of the magnetic phase developing therein. We argue that magnetism in Eu2Ir2O7 develops based on the nucleation of magnetic droplets at TN , whose successive growth is limited by the need of a continuous generation of magnetic hedgehog monopoles.
Searching for a Kitaev spin liquid phase motivated intense research on the honeycomb iridate mate... more Searching for a Kitaev spin liquid phase motivated intense research on the honeycomb iridate materials. However, access to a spin liquid ground state has been hindered by magnetic ordering. Cu2IrO3 is a new honeycomb iridate without thermodynamic signatures of a long-range order. Here, we use muon spin relaxation to uncover the magnetic ground state of Cu2IrO3. We find a two-component depolarization with slow and fast relaxation rates corresponding to distinct regions with dynamic and static magnetism coexisting in the same sample. X-ray absorption spectroscopy and first principles calculations identify a mixed copper valence as the origin of this behavior. Our results suggest that a minority of Cu 2+ ions nucleate regions of static magnetism whereas the majority of Cu + /Ir 4+ on the honeycomb lattice give rise to a Kitaev spin liquid.
High carrier density quantum wells embedded within a Mott insulating matrix present a rich arena ... more High carrier density quantum wells embedded within a Mott insulating matrix present a rich arena for exploring unconventional electronic phase behavior ranging from non-Fermi-liquid transport and signatures of quantum criticality to pseudogap formation. Probing the proposed connection between unconventional magnetotransport and incipient electronic order within these quantum wells has however remained an enduring challenge due to the ultra-thin layer thicknesses required. Here we address this challenge by exploring the magnetic properties of high-density SrTiO3 quantum wells embedded within the antiferromagnetic Mott insulator SmTiO3 via muon spin relaxation and polarized neutron reflectometry measurements. The one electron per planar unit cell acquired by the nominal d0 band insulator SrTiO3 when embedded within a d1 Mott SmTiO3 matrix exhibits slow magnetic fluctuations that begin to freeze into a quasistatic spin state below a critical temperature T*. The appearance of this quasi...
When electrons are confined in two-dimensional materials, quantum-mechanical transport phenomena ... more When electrons are confined in two-dimensional materials, quantum-mechanical transport phenomena and high mobility can be observed. Few demonstrations of these behaviours in surface spin-orbit bands exist. Here, we report the observation of quantized conductance in the surface bands of 50-nm Bi nanowires. With increasing magnetic fields oriented along the wire axis, the wires exhibit a stepwise increase in conductance and oscillatory thermopower, possibly due to an increased number of high-mobility spiral surface modes based on spin-split bands. Surface high mobility is unexpected since bismuth is not a topological insulator and the surface is not suspended but in contact with the bulk. The oscillations enable us to probe the surface structure. We observe that mobility increases dramatically with magnetic fields because, owing to Lorentz forces, spiral modes orbit decreases in diameter pulling the charge carriers away from the surface. Our mobility estimates at high magnetic fields ...
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Papers by Michael J Graf