Conductance of the edge modes and conductance across the copropagating edge modes around the $\nu =4/3$, 5/3, and 2 quantum Hall states are measured by individually exciting the modes. Temperature-dependent equilibration rates of the outer unity conductance edge mode are presented for different filling fractions. We find that the equilibration rate of the outer unity conductance mode at $\nu =2$ is higher and more temperature sensitive compared to the modes at fractional fillings 5/3 and 4/3. At the lowest temperature, the equilibration length of the outer unity conductance mode tends to saturate with the lowering filling fraction $\nu$ by increasing the magnetic field $B$. We speculate this saturating nature of the equilibration length is arising from an interplay of the Coulomb correlation and spin orthogonality.

Surface segregation is a phenomenon that depends on the delicate interplay between thermodynamic driving forces and kinetic obstacles, for which elevated temperature is often needed to enhance the atom mobility and reach equilibrium. Using the classic system of ${\mathrm{Cu}}_3\mathrm{Au}\left(100\right)$ under nonisothermal conditions, herein, we show an adatom process underlying transient surface segregation dynamics through the temperature-change-driven creation and annihilation of thermal vacancies in the bulk and the resulting bulk-surface mass exchanges. This is demonstrated by monitoring the surface composition evolution of ${\mathrm{Cu}}_3\mathrm{Au}\left(100\right)$ with temperature changes between 250 and $500{}^{\circ }\mathrm{C}$, showing that the increase in temperature decreases monotonically the surface Au concentration as a result of the transfer of more Cu than Au from the bulk to the surface to form Cu-rich clusters of adatoms. Such a bulk thermal defect effect is expected to be universal in inducing the disparity in the bulk-surface mass exchanges of dissimilar atoms in multicomponent materials because of the inherent differences in the vacancy formation energies of the constituent atoms.

We compute the Casimir force between two parallel and infinitely long carbon nanotubes. The force induced by fluctuations of the electromagnetic field follows from the Maxwell stress tensor, expressed in terms of the correlators of the fields which are obtained from the Green's function and the fluctuation-dissipation theorem for the current density. We show how the force depends on the distance between the nanotubes, as well as on their chiralities and semiconducting or metallic natures. We also analyze the convergence of the approximation we use that replaces integrals in frequency by sums over complex Matsubara frequencies. The methodology presented and the results obtained can contribute to the understanding of how electromagnetic fluctuations induce Casimir forces in complex microstructured geometries.

We present an optical spectroscopy analysis of the molecular beam epitaxy-grown InAs/GaSb quantum systems to study the higher-order optical transitions in InAs/GaSb superlattices (SLs). We have investigated two type-II SL structures with different layer widths: $\mathrm{In}{\mathrm{As}}_{8\mathrm{ML}}/\mathrm{Ga}{\mathrm{Sb}}_{8\mathrm{ML}}$ and $\mathrm{In}{\mathrm{As}}_{10\mathrm{ML}}/\mathrm{Ga}{\mathrm{Sb}}_{10\mathrm{ML}}$. To experimentally determine the spectral distribution of the higher-order transitions, the Fourier-transformed photoluminescence and Fourier-transformed photomodulated reflectance measurements were used. These measurements were undertaken vs temperature and incident polarization of the probing beam. Complementary theoretical calculations based on the eight $\mathbf{k}·\mathbf{p}$ formalism were also performed to identify the electron and heavy and light hole energy band ladder. Polarization-dependent measurements indicated the interchanging nature of the light and heavy hole states for the second and third valence band vs the SL wave vector $\left(k_z\right)$ along the growth direction. The results were compared with other calculations based on different models and existing experimental results, then summarized and reviewed for possible numerical parameters application ranges in the interface modeling.

Dissipation in the quantum solid of graphene is couched in the methodology of nonequilibrium statistical mechanics of an open quantum system. Of specific interest is the absorption accompanied by relaxation of energy because of an external frequency-dependent oscillatory electric field. At optical frequencies and for sizable amplitude of the electric field, the response is nonlinear, the treatment of which is simplified by means of a “rotating wave approximation” in which rapidly changing, off-resonance terms are omitted. The characteristic resonant frequency is what quantifies the tunneling between the valence and the conduction bands across the so-called Dirac point in graphene. The valence and the conduction states are mapped into the eigenstates of pseudo Pauli spin operators and the corresponding Hamiltonian, when embedded in a dissipative heat bath comprising the surrounding phonons and other electrons, makes possible a comprehensive analysis in terms of the much-studied spin-boson model of dissipative quantum statistical mechanics. A master equation for the density operator associated with this Hamiltonian yields rate equations for the mean population of the valence and the conduction states as well as the transition (matrix elements) between them. Further approximation of these rate equations allows contact with phenomenological treatments of the nonlinear optical conductivity. The present paper then provides a microscopic framework for investigating the response characteristics of a material of great topical interest using contemporary methods of quantum dissipation.

Using ab initio density functional theory, here we systematically study the monolayer ${\mathrm{MoOCl}}_2$ with a $4d^2$ electronic configuration. Our main result is that an orbital-selective Peierls phase (OSPP) develops in ${\mathrm{MoOCl}}_2$, resulting in the dimerization of the Mo chain along the $b$ axis. Specifically, the Mo-$d_{xy}$ orbitals form robust molecular-orbital states inducing localized $d_{xy}$ singlet dimers, while the Mo-$d_{xz/yz}$ orbitals remain delocalized and itinerant. Our study shows that ${\mathrm{MoOCl}}_2$ is globally metallic, with the Mo-$d_{xy}$ orbital bonding-antibonding splittings opening a gap and the Mo-$d_{xz/yz}$ orbitals contributing to the metallic conductivity. Overall, the results resemble the recently much discussed orbital-selective Mott phase but with the localized band induced by a Peierls distortion instead of Hubbard interactions. Finally, we also qualitatively discuss the possibility of OSPP in the $3d^2$ configuration, as in ${\mathrm{CrOCl}}_2$.

For spin rotational symmetric models with a positive-definite high-temperature expansion of the partition function, a stochastic sampling of the series expansion upon partial resummation becomes logically equivalent to sampling an uncolored closely packed loop-gas model in one higher dimension. Based on this, we devise quantum Monte Carlo updates that importance-sample loop configurations for general $SU\left(N\right)$ in fundamental and higher-symmetric representations. The algorithmic performance systematically improves with an increase in (continuous) $N$, allowing the efficient simulation of quantum paramagnets. The underlying reason for the increased efficacy is the correspondence of quantum paramagnetic phases such as valence bond solids to short-loop phases on the loop-gas side rather than the particular value of $N$. This also gives a connection between Sandvik's $JQ$ model class and classical loop-gas models in the deconfined universality class.

Hybrid Ising superconductor-ferromagnetic insulator heterostructures provide a unique opportunity to explore the interplay between proximity-induced magnetism, spin-orbit coupling, and superconductivity. Here we use a combination of first-principles calculations of ${\mathrm{NbSe}}_2/{\mathrm{CrBr}}_3$ heterostructures and an analytical theory of Ising superconductivity to analyze the existing experiments and provide a complete explanation of highly nontrivial and largely counterintuitive effects: an increase in the magnitude of the superconducting gap accompanied by the broadening of the tunneling peaks; hysteretic behavior of the tunneling conductance that sets in $\approx 2$ K below $T_c$; and nematic symmetry breaking in the superconducting state. The microscopic reason in all three cases appears to be the interplay between the proximity-induced exchange splitting and intrinsic defects. Finally, we predict additional interesting effects that at the moment cannot be addressed experimentally: spin-filtering when tunneling across ${\mathrm{CrBr}}_3$ and tunneling “hot spots” in momentum space that are anticorrelated with regions where the spin-orbit splitting is maximum.

Topological states of matter were first introduced for noninteracting fermions on an infinite uniform lattice. Since then, substantial effort has been made to generalize these concepts to more complex settings. Recently, local markers have been developed that can describe the topological state of systems without translational symmetry and a well-defined gap. However, no local marker for interacting matter has been proposed yet that is capable of directly addressing an interacting system. Here, we suggest such a many-body local marker based on the single-particle Green's function. Using this marker we identify topological transitions in finite lattices of a Chern insulator with Anderson disorder and Hubbard interactions. Importantly, our proposal can be generalized to nonequilibrium systems.

We present a comprehensive methodology to enable the addition of van der Waals (vdW) corrections to machine learning (ML) atomistic force fields. Using a Gaussian approximation potential (GAP) [Bartók et al., Phys. Rev. Lett. 104, 136403 (2010)] as a baseline, we accurately machine learn a local model of atomic polarizabilities based on Hirshfeld volume partitioning of the charge density [Tkatchenko and Scheffler, Phys. Rev. Lett. 102, 073005 (2009)]. These environment-dependent polarizabilities are then used to parametrize a screened London-dispersion approximation to the vdW interactions. Our ML vdW model only needs to learn the charge density partitioning implicitly by learning the reference Hirshfeld volumes from density functional theory (DFT). In practice, we can predict accurate Hirshfeld volumes from the knowledge of the local atomic environment (atomic positions) alone, making the model highly computationally efficient. For additional efficiency, our ML model of atomic polarizabilities reuses the same many-body atomic descriptors used for the underlying GAP learning of bonded interatomic interactions. We also show how the method enables straightforward computation of gradients of the observables, even when these remain challenging for the reference method (e.g., calculating gradients of the Hirshfeld volumes in DFT). Finally, we demonstrate the approach by studying the phase diagram of ${\mathrm{C}}_{60}$, where vdW effects are important. The need for a highly accurate vdW-inclusive reactive force field is highlighted by modeling the decomposition of the ${\mathrm{C}}_{60}$ molecules taking place at high pressures and temperatures.

${\mathrm{Ga}}_2{\mathrm{O}}_3$ is a wide-band-gap semiconductor of great interest for applications in electronics and optoelectronics. Two-dimensional (2D) ${\mathrm{Ga}}_2{\mathrm{O}}_3$ synthesized from top-down or bottom-up processes can reveal new heterogeneous structures and promising applications. In this paper, we study phase transitions among three low-energy stable ${\mathrm{Ga}}_2{\mathrm{O}}_3$ monolayer configurations using density functional theory and a machine learning Gaussian approximation potential, together with solid-state nudged elastic band calculations. Kinetic minimum energy paths involving direct atomic jump as well as concerted layer motion are investigated. The low phase transition barriers indicate feasible tunability of the phase transition and orientation via strain engineering and external electric fields. Large-scale calculations using the trained machine learning potential on the thermally activated single-atom jumps reveal the clear nucleation and growth processes of different domains. The results provide useful insights into future experimental synthesis and characterization of 2D ${\mathrm{Ga}}_2{\mathrm{O}}_3$ monolayers.

The combination of topology and magnetism is attractive to produce exotic quantum matters, such as the quantum anomalous Hall state, axion insulators, and the magnetic Weyl semimetals. $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$, as an intrinsic magnetic topological insulator, provides a platform for the realization of various topological phases. Here we report the intermediate Hall steps in the magnetic hysteresis of $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$, where four distinguishable magnetic memory states at zero magnetic field are revealed. The gate and temperature dependence of the magnetic intermediate states indicates the noncollinear spin structure in $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$, which can be attributed to the Dzyaloshinskii-Moriya interaction as the coexistence of strong spin-orbit coupling and local inversion symmetry breaking on the surface. Moreover, these multiple magnetic memory states can be programmatically switched among each other through applying designed pulses of magnetic field. Our results provide some insights of the influence of bulk topology on the magnetic states, and the multiple memory states should be promising for spintronic devices.

We theoretically consider spin pumping in a junction between a ferromagnetic insulator (FI) and a two-dimensional electron gas (2DEG) in which the Rashba and Dresselhaus spin-orbit interactions coexist. Using second-order perturbation theory, we derive an increase in linewidth in the case of an interfacial exchange coupling in a ferromagnetic resonance (FMR) experiment. We clarify how the enhancement of Gilbert damping depends on the resonant frequency and spin orientation of the FI. We show that this setup of an FMR experiment can provide information on the spin texture of 2DEG at the Fermi surface.

Search of high-temperature superconductors has gained huge impetus since the discovery of superconductivity in bulk $\mathrm{Mg}{\mathrm{B}}_2$. These efforts led to the synthesis of high-$T_C$ materials in the megabar pressure region. However, the ultimate goal of a room-conditions superconductor is still elusive. Toward this, a class of two-dimensional (2D) superconductors is emerging as a fertile field of research. In this paper, by solving fully anisotropic Migdal-Eliashberg equations, we show that a pristine monolayer (ML) of LiBC will be a Bardeen-Cooper-Schrieffer-type superconductor with a record-breaking $T_C$ of 70 K among pure 2D superconductors. The critical temperature could be further increased by hydrogenation of the ML. Analysis of the electronic properties indicates the partial change of B-C covalent bonding from $s{p}^2$ to $s{p}^3$ type on bulk-to-ML transformation. This paper presents a proposal to metalize the LiBC system, which was long been predicted to show superconductivity in its bulk form with 50% Li site vacancies. This system might be useful for the design and development of high-$T_C$ 2D superconductors that could be applied in devices like quantum interferometers, superconducting qubits, or superconducting transistors.

Experimental and theoretical studies on superconductivity in ${\mathrm{SrIr}}_2$ and ${\mathrm{SrRh}}_2$ Laves phases are presented. The measured resistivity, heat capacity, and magnetic susceptibility confirm the superconductivity of these compounds with $T_c=6.07$ and 5.41 K, respectively. Electronic structure calculations show that the Fermi surface is mostly contributed by $5d$ ($4d$) electrons of Ir (Rh), with Sr atoms playing the role of electron donors. The effect of the spin-orbit coupling is analyzed and found to be important in both materials. Lattice dynamics and electron-phonon coupling (EPC) are studied and the strong electron-phonon interaction is found, contributed mostly by the low-frequency Ir and Rh vibrations. The enhancement of EPC, when compared to weakly coupled metallic Ir and Rh, is explained by the strong modifications in the propagation of phonons in the network of Ir (Rh) tetrahedrons, which are the building blocks of the Laves phase, and originate from the metallic fcc structures of elemental iridium and rhodium.

The discovery of topological superconductivity in doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ made this class of materials highly important for the field of condensed matter physics. However, the structural origin of the superconducting state remained elusive, despite being investigated intensively in recent years. We use scanning tunneling microscopy and the normal incidence x-ray standing wave (NIXSW) technique in order to determine the vertical position of the dopants—one of the key parameters for understanding topological superconductivity in this material— for the case of ${\mathrm{Sr}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$. In particular, we analyze the NIXSW data in consideration of the inelastic mean free path of the photoemitted electrons, which allows us to distinguish between symmetry-equivalent sites. We find that Sr atoms are not situated inside the van der Waals gap between the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ quintuple layers but rather in the quintuple layer close to the outer Se planes.

In this paper we are motivated to numerically investigate the fundamental strong-field and attosecond response processes in wide band-gap solids, using the first-principle approach based on time-dependent density functional theory, advancing our understanding of fundamental strong-field and attosecond physics in solids. Taking cubic silicon carbide as a concrete example, the influence of pulse parameters on the ultrafast responses is demonstrated, which allows one to gain a useful independent insight into the interaction processes of attosecond light pulses with solids, revealing information currently not accessible by experiment. We also demonstrate control of the charge transfer and dielectric permittivity by intense attosecond pulse light. It is concluded that a single-cycle attosecond pulse light can be used to effectively manipulate the charge transfer and dielectric properties of wide-band gap materials with great potential for ultrafast device applications.

Quasiperiodic systems serve as fertile ground for studying localization, due to their propensity already in one dimension to exhibit rich phase diagrams with mobility edges. The deterministic and strongly correlated nature of the quasiperiodic potential nevertheless offers challenges distinct from disordered systems. Motivated by this, we present a theory of localization in quasiperiodic chains with nearest-neighbor hoppings, based on the convergence of local propagators; exploiting the fact that the imaginary part of the associated self-energy acts as a probabilistic order parameter for localization transitions and, importantly, admits a continued-fraction representation. Analyzing the convergence of these continued fractions, localization or its absence can be determined, yielding in turn the critical points and mobility edges. Interestingly, we find anomalous scalings of the order parameter with system size at the critical points, consistent with the fractal character of critical eigenstates. Self-consistent theories at high orders are also considered, shown to be conceptually connected to the theory based on continued fractions, and found in practice to converge to the same result. Results are exemplified by analyzing the theory for three quasiperiodic models covering a range of behavior.

The electronic and magnetic structure, including the Heisenberg model exchange interaction parameters, was explored for the recently proposed novel cuprate ${\mathrm{Cu}}_2{\mathrm{F}}_5$. Using the $\mathrm{DFT}+\mathrm{U}$ calculation, it is shown that the compound is formed by two types of copper ions with $d^9$ and $d^8$ electronic configurations. We have found a very stable antiferromagnetic ordering with strong anisotropy of exchange interaction that results in the appearance of an unusual 2D-magnetism: within the (100)-plane the exchange between the S = 1 and S = 1/2 Cu ions has almost the same strength as between the two S = 1 ions. The interplane magnetic interaction is five times weaker than the in-plane one.

The structural and concentration conditionality of magnetic states and transport properties of layered chalcogenide compounds ${\mathrm{Fe}}_x{\mathrm{TiS}}_2$ ($0\le x\le 0.75$) have been studied by means of x-ray and neutron diffraction, magnetization, electrical resistivity, and magnetoresistance measurements performed on polycrystalline samples synthesized by solid-phase reaction method with prolonged homogenization heat treatment. It has been revealed that various magnetic states [spin-cluster glass state at $x<0.25$, antiferromagnetic (AFM) order at $x\approx 0.25–0.28$, cluster glass state at $x\approx 0.33$, AFM at $x\approx 0.45–0.5$, and ferrimagnetic ordering above $x=0.5$] are realized in the $\mathrm{Fe}{}_x\mathrm{TiS}{}_2$ system with increasing Fe content. The absence of a long-range magnetic order in ${\mathrm{Fe}}_{0.33}{\mathrm{TiS}}_2$ is confirmed by neutron diffraction measurements. The magnetic states observed in this system are closely related not only to the concentration of Fe atoms, but also, to a greater extent, to their distribution over the cation layers. It is assumed that the single crystals obtained by chemical vapor deposition are practically unique objects with only their inherent properties, which leads to inconsistency between the literature data on the magnetic properties of these compounds. Changes in the magnetic state of ${\mathrm{Fe}}_x{\mathrm{TiS}}_2$ with Fe concentration are accompanied by nonmonotonic dependencies of the magnetoresistance and coercivity with maximal absolute values in compounds with the AFM virgin state. The ferrimagnetic order in compounds with $x>0.5$ is suggested to originate in Fe-Ti mixing in cationic layers. The AFM ordered compounds undergo the field-induced phase transitions to the metastable high-coercive ferromagnetic (FM) state, which is accompanied by a large remnant magnetoresistance. The enhanced coercivity observed in compounds with the AFM virgin state is ascribed to the intrinsic exchange bias together with the Ising-type spin state of Fe ions.