Low Dimensional Systems

Strongly correlated matter have been studied since the first appearance of insulating behavior in
believed conductors. The first model properly capturing electronic correlations, the Hubbard
model, changed the ways we think about electrons in crystals. This become even more compelling
due to emergent novel phenomena cased by the competition of electronic, spin and orbital degrees
of freedom. Iron-based superconductors are a prominent example of such unique physics. Recent
inelastic neutron scattering experiments on iron-based ladders revealed the existence of the ↑↑↓↓
magnetic state. As magnetic correlations are believed to be of utmost importance in the study of
high-Tc superconductivity (SC), a considerable focus was put on the magnetic normal phase in
such systems. Years of both theoretical and experimental investigations probed magnetism in
low-dimensional high-Tc SC. However, the finite temperature dynamics were put aside as the
numerical complexity is tremendous. This study focuses on exact numerical techniques to capture
the whole Hilbert space, thus the calculation of thermodynamic quantities is straightforward,
unlike in approximate techniques. Although only small clusters can be studied, I present novel
results for multiorbital systems, modeled by the generalized Kondo-Heisenberg model. We show
evidence of the Coulomb assisted quantum phase transition from a singlet to triplet ground state.
Additionally we hint the possibility of fractional excitations and a spin-liquid phase at elevated
interaction strength.

In this paper we discuss the effect of dissipated power on the characteristic time of noise spectra transformation investigated in wide-bandgap AlGaN/GaN transistor structures with long channels. It is found that the characteristic time as a function of temperature demonstrates an exponential dependence with definite activation energy. Obtained results are explained based on the developed model of non-equilibrium fluctuations of the sample resistance. These fluctuations cause a local overheating due to local change of resistance. In the long channels these fluctuations may create overheating regions with two different self-heating temperatures, leading to different activation dependences, observed in the experiment.

Tsallis q-extension of statistics and fractal generalization of dynamics are two faces of the same physical reality, as well as the Kernel modern complexity theory. The fractal generalization dynamics is based at the multiscale – multifractal characters of complex dynamics in the physical space-time and the complex system’s dynamical phase space. Tsallis q-triplet of non-extensive statistics can be used for the experiment test of q-statistic as well as of the fractal dynamics. In this study we present indicative experimental verifications of Tsallis theory in various complex systems such as solar plasmas, (planetic magnetospheres, cosmic stars and cosmic rays), atmospheric dynamics, seismogenesis and brain dynamics.

The ideal uniform two-dimensional (2D) Fermi and Bose gases are considered both in the thermodynamic limit and the finite case. We derive May's Theorem, viz. the correspondence between the internal energies of the Fermi and Bose gases in the thermodynamic limit. This results in both gases having the same heat capacity. However, as we shall show, the thermodynamic limit is never truly reached in two dimensions and so it is essential to consider finite-size effects. We show in an elementary manner that for the finite 2D Bose gas, a pseudo-Bose-Einstein condensate forms at low temperatures, incompatible with May's Theorem. The two gases now have different heat capacities, dependent on the system size and tending to the same expression in the thermodynamic limit.

We report the results of direct measurements and a theoretical investigation of the in-plane effective mass in the two-dimensional electron gas of nominally undoped AlGaN/GaN heterostructures with a different degree of quantum confinement. It is shown that in most cases the conduction band nonparabolicity effect is overestimated and the electron wave-function penetration into the barrier layer should be taken into account. The contribution of the wave-function hybridization is determined to play the dominant role. The band edge effective mass value is deduced to be (0.2+-0.01) m0.

Growth of Langmuir-Blodgett ͑LB͒ films of nickel arachidate ͑NiA͒ on differently terminated ͑OH-, H-, or Br-terminated͒ Si͑001͒ substrates and their structural evolution with time have been investigated by x-ray reflectivity technique and complemented by atomic force microscopy. Stable and strongly attached asymmetric monolayer ͑AML͒ of NiA is found to grow on freshly prepared oxide-covered Si substrate while unstable and weakly attached symmetric monolayer ͑SML͒ of NiA grows on H-terminated Si substrate, corresponding to stable hydrophilic and unstable hydrophobic natures of the substrates, respectively. The structure of LB film on Br-terminated Si substrate, however, shows intermediate behavior, namely, both AML and SML are present on the substrate, indicative of coexisting ͑hydrophilic and hydrophobic͒ nature of this terminated surface. Such coexisting nature of the substrate shows unusual growth behavior of LB films: ͑i͒ hydrophilic and hydrophobic attachments of NiA molecules in single up stroke of deposition and ͑ii͒ growth of few ring-shaped largeheights islands in subsequent deposition. These probably occur due to the presence of substrate-induced perturbation in the Langmuir monolayer and release of initially accumulated strain in the film structures near hydrophilic/hydrophobic interface, respectively, and provide the possibility to grow desired structures ͑AML or SML͒ of LB films by passivation-selective surface engineering.

Ga 1−x N / GaN heterostructures was studied and compared with PL spectra of GaN films grown on sapphire substrates. It is demonstrated that optical emission in the energy range of 3.3-3.46 eV related to the two-dimensional electron gas radiative processes can be completely suppressed in modulation-doped Al x Ga 1−x N / GaN heterostructures. Instead of this, an intense broad long-wavelength emission attributed to the recombination of donor-acceptor pairs in the lower energy range of 2.7-3.3 eV is revealed. This spectral transformation is explained by the presence of deep-level defect-related acceptor centers in Al x Ga 1−x N / GaN heterostructures introduced at the modulation doping of the Al x Ga 1−x N barrier layer.

In this paper we discuss the effect of dissipated power on the characteristic time of noise spectra transformation investigated in wide-bandgap AlGaN/GaN transistor structures with long channels. It is found that the characteristic time as a function of temperature demonstrates an exponential dependence with definite activation energy. Obtained results are explained based on the developed model of non-equilibrium fluctuations of the sample resistance. These fluctuations cause a local overheating due to local change of resistance. In the long channels these fluctuations may create overheating regions with two different self-heating temperatures, leading to different activation dependences, observed in the experiment.

One-dimensional wires are known to be inherently unstable at finite temperature. Here, we show that long-range order of atomic Au double chains adsorbed on a Si(553) surface is not only stabilized by interaction with the substrate, but spontaneous self-healing of structural defects is actually enforced by the adsorption of atomic species such as Au or H. This is true even for random adsorbate distribution. Combining atomistic models within density functional theory with low energy electron diffraction and high-resolution electron energy loss spectroscopy, we demonstrate that this apparently counterintuitive behavior is mainly caused by adsorption-induced band filling of modified surface bands, i.e., by the strong electronic correlation throughout the whole terrace. Although adsorption preferably occurs at the step edge, it enhances the dimerization and the stiffness of the Au dimers. Thus, the intertwinement of quasi-1D properties with delocalized 2D effects enforces the atomic wire order. The reduction of size and dimensionality of metallic objects gives rise to peculiar physical properties such as quantization of conductance, spin-charge separation, charge and spin density waves, and Luttinger liquid behavior [1-5], which are subject of fundamental studies on low-dimensional physics. Unfortunately, free-standing metallic wires cannot be realized due to their inherent instability [5]. This problem can be partially circumvented by the formation of self-assembled, metallic atomic chains on stepped semiconductor surfaces such as Ge and Si, which are regarded as the low-end limit for long-range ordered nano-structures [6-15]. Several realizations of such wire systems have been reported [11,16-21]. While structural embedding is a prerequisite for the realization of 1D wires, the coupling to higher dimensions implies an intricate balance between preservation and destruction of typical 1D instabilities [22]. On the other hand, the coupling to higher dimensions can be exploited to tune wire properties such as morphology and conductivity. Here, we demonstrate a quite unique and unexpected example thereof: the enhanced and even enforced order of atomic chains by random adsorption of atoms, as a new mechanism of crossover between 1D and 2D properties. This behavior is in striking contrast to previously investigated adsorbates, which adsorb at random for low concentrations and thus introduce disorder. While an increased order might be expected if the adsorbates build patterns with the same periodicity of existing structures on the surface, it appears counterintuitive as a consequence of randomly distributed adatoms. Among the metallic wires grown on regularly stepped surfaces, the Si(553)-Au is one of the most studied systems, due to its relatively high stability, and because of a long-standing debate concerning its exact atomic structure [11,18,19,23-27]. It is commonly accepted that the system features a Si honeycomb and a Au double chain on each terrace. The Au chain is dimerized (even in the absence of adsorbates), which leads to a ×2 periodicity along the terraces. Several attempts to tune the conductivity of this system by deposition of oxygen [28,29] or hydrogen [30-32] are available in the literature. They demonstrate an intricate scenario with adsorbate specific modifications of the electronic structure. Generally, different adsorption sites of similar energy are available on the surface, leading to a random adsorbate distribution and a complex modification of the electronic structure. In this Letter, we demonstrate the improvement of the dimerized Au chain order simply by adsorption of small amounts of surplus Au, or of atomic H at room temperature. Since these adsorbates, as shown in the following, adsorb at random sites, this effect does not depend on the formation of ordered adsorbate arrays. Density functional theory (DFT) calculations show that the observed behavior stems from the coupling of the 1D atomic chains to higher dimensions. The electronic structure is modified by the effective electron donation from the adsorbates. The latter affects nonlocally the atomic arrangement of the whole terrace, demonstrating the extreme electronic correlation in this quasi-1D system. The rearranged structure is by far less PHYSICAL REVIEW LETTERS 126, 106101 (2021)

We have calculated the magnetic moment per atom, Image of symmetric nickel cluster-dimers NiN–NiN as a function of cluster–cluster distance D and cluster size N. The spin-polarized electronic structure has been calculated with a self-consistent tight-binding method considering the 3d, 4s and 4p valence electrons. We have analyzed the partial sp and d contributions to Image The d component shows a monotonic behavior and provides the dominant contribution to Image whereas the sp contribution shows a non-monotonic (and complex) behavior as a function of the distance and of cluster size. The approaching clusters change their intrinsic magnetic moments at separations of the order of the bulk first nearest-neighbor distance dfn. For N=5–7 there is a range of separations (1dfn–3dfn) where the cluster moments are slightly enhanced.

The MPX 3 family of magnetic van-der-Waals materials (M denotes a first row transition metal and X either S or Se) are currently the subject of broad and intense attention for low-dimensional magnetism and transport and also for novel device and technological applications, but the vanadium compounds have until this point not been studied beyond their basic properties. We present the observation of an isostructural Mott insulator-metal transition in van-der-Waals honeycomb antiferromagnet V 0.9 PS 3 through highpressure x-ray diffraction and transport measurements. We observe insulating variable-range-hopping type resistivity in V 0.9 PS 3 , with a gradual increase in effective dimensionality with increasing pressure, followed by a transition to a metallic resistivity temperature dependence between 112 and 124 kbar. The metallic state additionally shows a low-temperature upturn we tentatively attribute to the Kondo effect. A gradual structural distortion is seen between 26 and 80 kbar, but no structural change at higher pressures corresponding to the insulator-metal transition. We conclude that the insulator-metal transition occurs in the absence of any distortions to the lattice-an isostructural Mott transition in a new class of two-dimensional material, and in strong contrast to the behavior of the other MPX 3 compounds.

Photoluminescence PL in modulation-doped and nominally undoped AlxGa1−xN/GaN heterostructures was studied and compared with PL spectra of GaN films grown on sapphire substrates. It is demonstrated that optical emission in the energy range of 3.3 - 3.46 eV related to the two-dimensional electron gas radiative processes can be completely suppressed in modulation-doped AlxGa1−xN/GaN heterostructures. Instead of this, an intense broad long-wavelength emission attributed to the recombination of donor-acceptor pairs in the lower energy range of 2.7 - 3.3 eV is revealed. This spectral transformation is explained by the presence of deep-level defect-related acceptor centers in AlxGa1−xN/GaN heterostructures introduced at the modulation doping of the AlxGa1−xN barrier layer.

Selective spatial confinement of the electric field intensity is theoretically obtained for the light propagation along hybrid structures conformed by periodic and Fibonacci quasiperiodic dielectric multilayers. Sandwich-like configurations featuring periodic-quasiperiodic-periodic as well as quasiperiodic-periodic-quasiperiodic designs exhibit spatial localization of a large percent of the optical signal within specific zones of the hybrid system. Such a feature might be of interest in the pursuing of lasing devices based on porous silicon. It is found that the electric field confinement does not only depend on the quality of the defect or a particular transmission mode observed in the reflectivity spectra. We show that it is possible to enhance the electric field confinement solely varying the angle of incidence. The possibility of realizing finite photonic crystals with reduced size and very well defined band gap by means of a quasiperiodic-periodicquasiperiodic hybrid multilayer is also revealed.

The understanding of the dynamical properties of skyrmion is a fundamental aspect for the realization of a competitive skyrmion based technology beyond CMOS. Most of the theoretical approaches are based on the approximation of a rigid skyrmion. However, thermal fluctuations can drive a continuous change of the skyrmion size via the excitation of thermal modes. Here, by taking advantage of the Hilbert-Huang transform, we demonstrate that at least two thermal modes can be excited which are non-stationary in time. In addition, one limit of the rigid skyrmion approximation is that this hypothesis does not allow for correctly describing the recent experimental evidence of skyrmion Hall angle dependence on the amplitude of the driving force, which is proportional to the injected current. In this work, we show that, in an ideal sample, the combined effect of field-like and damping-like torques on a breathing skyrmion can indeed give rise to such a current dependent skyrmion Hall angle. While here we design and control the breathing mode of the skyrmion, our results can be linked to the experiments by considering that the thermal fluctuations and/or disorder can excite the breathing mode. We also propose an experiment to validate our findings.

In this study which is the continuation of the first part (Pavlos et al. 2012) [1], the nonlinear analysis of the solar flares index is embedded in the non-extensive statistical theory of Tsallis (1988) [3]. The q-triplet of Tsallis, as well as the correlation dimension and the
Lyapunov exponent spectrum were estimated for the singular value decomposition (SVD) components of the solar flares timeseries. Also the multifractal scaling exponent spectrum f (a), the generalized Renyi dimension spectrum D(q) and the spectrum J(p) of the structure
function exponents were estimated experimentally and theoretically by using the q-entropy principle included in Tsallis non-extensive statistical theory, following Arimitsu and Arimitsu (2000) [25]. Our analysis showed clearly the following: (a) a phase transition
process in the solar flare dynamics from a high dimensional non-Gaussian self-organized critical (SOC) state to a low dimensional also non-Gaussian chaotic state, (b) strong intermittent solar corona turbulence and an anomalous (multifractal) diffusion solar corona
process, which is strengthened as the solar corona dynamics makes a phase transition to low dimensional chaos, (c) faithful agreement of Tsallis non-equilibrium statistical theory with the experimental estimations of the functions: (i) non-Gaussian probability distribution
function P(x), (ii) f (a) and D(q), and (iii) J(p) for the solar flares timeseries and its underlying non-equilibrium solar dynamics, and (d) the solar flare dynamical profile is revealed similar to the dynamical profile of the solar corona zone as far as the phase transition
process from self-organized criticality (SOC) to chaos state. However the solar low corona (solar flare) dynamical characteristics can be clearly discriminated from the dynamical characteristics of the solar convection zone.

Recently we have demonstrated a unique approach to optically pure, water-stable iron(II) metallo-helical assemblies, which they have termed flexicates. These system were shown to be active on the nanomolar range against a range of cancer cell lines and display antimicrobial activity against both Gram-negative bacterium E. coli (MC4100) and Gram-positive bacterium methicillin-resistant S. aureus (MRSA252).1, 2 Expanding on our previous systems we now propose a new class of bimetallic complexes we have termed triplex metallohelices. Theses highly stereoselective complexes have been shown both computationally and synthetically to adopt an anti-parallel arrangement of ligand strands affording the desired C1 symmetry typically associated with organic alpha-helixmimetics systems. Furthermore, these triplex metallohelices have been shown to be easily functionalised, water soluble and display anticancer properties in the nanomolar range against HCT116 p53++ (human colon carcinoma) cell lines.

1. V. Brabec, S. E. Howson, R. A. Kaner, R. M. Lord, J. Malina, R. M. Phillips, Q. M. A. Abdallah, P. C.
McGowan, A. Rodger and P. Scott, Chemical Science, 2013.
2. S. E. Howson, A. Bolhuis, V. Brabec, G. J. Clarkson, J. Malina, A. Rodger and P. Scott, Nat. Chem.,
2012, 4, 31-36.

Ga 1−x N / GaN heterostructures was studied and compared with PL spectra of GaN films grown on sapphire substrates. It is demonstrated that optical emission in the energy range of 3.3-3.46 eV related to the two-dimensional electron gas radiative processes can be completely suppressed in modulation-doped Al x Ga 1−x N / GaN heterostructures. Instead of this, an intense broad long-wavelength emission attributed to the recombination of donor-acceptor pairs in the lower energy range of 2.7-3.3 eV is revealed. This spectral transformation is explained by the presence of deep-level defect-related acceptor centers in Al x Ga 1−x N / GaN heterostructures introduced at the modulation doping of the Al x Ga 1−x N barrier layer.

The study of thermodynamics of topological defects is an important challenge to understand their underlying physics. Among them, magnetic skyrmions have a leading role for their physical properties and potential applications in storage and neuromorphic computing. In this paper, the thermodynamic statistics of magnetic skyrmions is derived. It is shown that the skyrmion free energy can be modelled via a parabolic function and the diameters statistics obeys the Maxwell-Boltzmann distribution. This allows for making an analogy between the behavior of the distribution of skyrmion diameters statistics and the diluted gas Maxwell-Boltzmann molecules distribution at thermodynamical equilibrium. The calculation of the skyrmion configurational entropy, due to thermally-induced changes of size and shape of the skyrmion, is essential for the determination of thermal fluctuations of the skyrmion energy around its average value. These results can be employed to advance the field of skyrmionics.

We studied electric current and noise in planar GaN nanowires (NWs). The results obtained at low voltages provide us with the estimates of the depletion effects in the NWs. For larger voltages, we observe the space-charge limited current (SCLC) effect. The onset of the effect clearly correlates with the NW width. For narrow NWs, the mature SCLC regime was achieved. This effect has great impact on fluctuation characteristics of studied NWs. At low voltages, we found that the normalized noise level increases with decreasing NW width. In the SCLC regime, a further increase in the normalized noise intensity (up to 10 4 times) was observed, as well as a change in the shape of the spectra with a tendency towards slope −3/2. We suggest that the features of the electric current and noise found in the NWs are of a general character and will have an impact on the development of NW-based devices.

We report on the electrical characterization of AlN/GaN/AlN double-barrier resonant tunnelling diodes (RTDs) using steady-state current-voltage and capacitance-voltage (C-V) characteristics in a wide frequency range with 2 kHz steps. The C-V characteristics of a double-barrier RTD show different behaviour under forward and reverse polarities and a strong dependence on frequency. The monotonous growth of capacitance at forward bias was registered, while a more complicated dependence was observed at reverse voltages. In order to analyse this dependence, a self-consistent calculation of the potential profile of the structure was performed taking into account polarization effects at the AlN/GaN interfaces. The peculiarities are analysed in the model of possible charge trapping at the interface states.

Understanding radiation effects on the mechanical properties of SiC composites is important to their application in advanced reactor designs. By means of molecular dynamics simulations, we found that due to strong interface bonding between the graphene layers and SiC, the sliding friction of SiC fibers is largely determined by the frictional behavior between graphene layers. Upon sliding, carbon displacements between graphene layers can act as seed atoms to induce the formation of single carbon atomic chains (SCACs) by pulling carbon atoms from the neighboring graphene planes. The formation, growth, and breaking of SCACs determine the frictional response to irradiation.

Evolution of interdiffused Gaussian-shape nanolayer of Au-Si, formed due to diffusion of Au into Si(111) substrate at ambient conditions, depends strongly on the Si surface pretreatment/passivation conditions. Negligible diffusion in the Au-OSi(111) sample, confirms the strong barrier action of the oxide-layer against diffusion, while large diffusion in the Au-HSi(111) sample compared to that in the Au-BrSi(111) sample suggests that the H-passivated Si surface is more stable. This nature of the Au-Si(111) system is qualitatively similar to that of the Au-Si(001) system but it differs quantitatively. The size, electronegativity and bond-energy of the passivating elements and the number of dangling bonds on the Si surface influence the instability of the Si surface. This instability, parameterized by growth-time of oxide layer alone, can be utilized to tune the amount of diffusion into the sub-surface Si region. The distribution of growth-time and fractional passivated area, which are related to the improper Si surface passivation, are against such control and needs perfection.

The growth and evolution of Ag nanolayers on differently-passivated Si͑001͒ substrates at ambient condition have been studied. Initial compactness and smoothness of Ag nanolayer on the H-passivated Si͑001͒ surface are found better compared to those on the Br-passivated Si͑001͒ surface, which can be understood considering surface free energy and surface mobility of the passivated surfaces. As the time passes, the growth of dewetted three-dimensional ͑3D͒ islandlike structures ͑Volmer-Weber-type mode͒ from comparatively wetted Ag nanolayer ͑Stranski-Krastanov-type mode͒ is evident at ambient conditions. Such evolution of growth is through dewetting ͑related to the change in the interfacial energy due to the oxide growth͒, migration, and coalesce of Ag, which can even produce large epitaxial ͓Ag͑001͒/Si͑001͔͒ 3D islands on H-passivated Si͑001͒ surface. The growth rate, size, number density, and epitaxy/nonepitaxy of 3D islands are different for different passivated surfaces. These differences can be realized considering the growth time of oxide ͑i.e., instability of passivated surface͒, in-plane inhomogeneity of interfacial energy ͑i.e., inhomogeneous nature of passivation͒, and in-plane diffusion of Ag on the passivated surfaces.