The classical map hypernetted-chain (CHNC) method for interacting electrons uses a kinetic energy functional in the form of a classical-fluid temperature. Here, we show that the CHNC generated two-body densities and pair-distribution functions (PDFs) correspond to $N$-representable densities. Comparisons of results from CHNC with quantum Monte Carlo (QMC) and path-integral Monte Carlo (PIMC) are used to validate the CHNC results. Since the PDFs are sufficient to obtain the equation of state or linear response properties of electron-ion systems, we apply the CHNC method for fully classical calculations of electron-ion systems in the quantum regime, using hydrogen at 4000 K and 350 times the solid density as an example since QMC comparisons are available. We also present neutral pseudoatom (NPA) calculations which use rigorous density functional theory (DFT) to reduce the many nuclear problem to an effective one-ion problem. The CHNC PDFs and NPA results agree well with the ion-ion, electron-ion, and electron-electron PDFs from QMC, PIMC, or DFT coupled to molecular dynamics simulations where available. The PDFs of a two-dimensional (2D) electron-hole system at 5 K are given as an example of 2D “warm dense” matter where the electrons and the counterparticles (holes) are all in the quantum regime. Basic methods such as QMC, PIMC, or even DFT become prohibitive while CHNC methods, being independent of the number of particles or the temperature, prove to be easily deployable.

The observation of quantized anomalous Hall conductance in the forced ferromagnetic state of $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ thin flakes has attracted much attention. However, a strong magnetic field is needed to fully polarize the magnetic moments due to the large antiferromagnetic interlayer exchange coupling. Here, we reported the magnetic and electrical transport properties of the magnetic van der Waals $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4{\left(\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3\right)}_n$ ($n=1,2$) single crystals, in which the interlayer antiferromagnetic exchange coupling is greatly suppressed with the increase of the separation layers $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$. $\mathrm{MnB}{\mathrm{i}}_4\mathrm{T}{\mathrm{e}}_7$ and $\mathrm{MnB}{\mathrm{i}}_6\mathrm{T}{\mathrm{e}}_{10}$ show weak antiferromagnetic transition at 12.3 and 10.5 K, respectively. The ferromagnetic hysteresis was observed at low temperature for both of the crystals, which is quite crucial for realizing the quantum anomalous Hall effect without external magnetic field. Our work indicates that $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4{\left(\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3\right)}_n$ ($n=1,2$) provides an ideal platform to investigate the rich topological phases with their two-dimensional limits.

The nature of the insulating and superconducting states in twisted bilayer graphene systems is intensely debated. While many works seek for explanations in the few flat bands near the Fermi level, theory and a number of experiments suggest that nontwisted bilayer graphene systems do exhibit — or are at least close to — an ordered, insulating ground state related to antiferromagnetic ordering. Here we investigate in which ways this magnetic ordering scenario is affected by the slight twisting between the layers. We find that at charge neutrality the ordering tendencies of twisted systems interpolate between those of untwisted AA and AB stacked bilayers at intermediate temperatures, while at lower temperatures of the order of typical flat-band dispersion energies, the ordering tendencies are even enhanced for the twisted systems. The preferred order at charge neutrality still exhibits an antiferromagnetic spin arrangement, with ordered moments alternating on the carbon-carbon bonds, with an enveloping variation on the moiré scale. This ordering can be understood as inherited from the untwisted systems. However, even in the random-phase approximation analysis, the possible low-energy behaviors are quite versatile, and slight doping of one or more electrons per moiré cell can take the system into a, potentially flat-band induced, ferromagnetic phase.

Robust topologically trivial low-energy Andreev bound states (ABSs) induced by position-dependent effective potentials have recently come under renewed focus in light of a remarkable set of experiments observing robust quantized zero-bias conductance plateaus in semiconductor-superconductor heterostructures. We show that (1) the partial spatial separation of the wave functions of the component Majorana bound states (MBSs) is crucial for the creation and stability of topologically trivial near-zero-energy Andreev bound states, (2) the signs of the spin polarizations of the component MBSs can be either the same or opposite, depending on the profile of the inducing potential, and (3) the spin polarizations do not play a fundamental role in generating vastly different coupling strengths to local probes and/or ensuring the robustness of the near-zero-energy ABS. Consequently, in contrast to recent theoretical claims (A. Vuik et al., arXiv:1806.02801) we find that a robust, quantized zero-bias conductance plateau of height $\sim 2e^2/h$ measured in the topologically trivial regime necessarily requires partially separated ABSs (ps-ABSs), independent of the relative signs of the spin polarizations. In addition, we show that (4) well-defined energy-splitting oscillations involve MBSs characterized by exponential tails pointing toward each other, and that (5) ps-ABSs generated by the tunnel barrier itself produce zero-bias conductance peaks with a characteristic width that increases strongly with the applied magnetic field. Finally, we propose (6) a quantitative scheme for analyzing the stability of Majorana modes based on probability distributions of splitting susceptibilities and show that a ps-ABS mode can be remarkably robust when judged based on its signature in a charge tunneling experiment, but, in essence, is topologically unprotected.

We study the thermoelectric transport properties of a three-dimensional massive relativistic fermion gas with screened Coulomb impurities in high magnetic fields where only the lowest Landau levels contribute to the transport. Our results can be applied to experimental results of gapless and gapped Dirac materials. We focus on the effects of the mass term and we show the main differences that arise compared to the massless Dirac fermions. The different behavior is shown to be relevant at higher magnetic fields. The calculations are performed in the framework of the linear response theory using the exact quantum mechanical solution of the system in a constant magnetic field. We prove that the Mott formula and the Wiedemann-Franz law are valid at low temperatures and use them to calculate the thermoelectric transport coefficients. We show that the temperature range where the low temperature approximation is valid increases with increasing magnetic fields. The magnetic field dependence of measurable quantities (i.e., conductivity, Seebeck coefficient, Nernst coefficient, and thermal conductivity) strongly depend on the magnetic field dependence of the scattering rate, thus the result relies on the proper treatment of the impurities. In this work they are included through the first Born approximation using screened charged impurities as impurity potential. We show that the electric conductivity does not change qualitatively in the case of finite mass term. On the other hand we find that the mass term causes significantly different behavior in the Seebeck and Nernst coefficients.

Semiconductor-superconductor hybrid systems provide a promising platform for hosting unpaired Majorana fermions towards the realization of fault-tolerant topological quantum computing. In this study we employ the Keldysh nonequilibrium Green's function formalism to model quantum transport in normal-superconductor junctions. We analyze III-V semiconductor nanowire Josephson junctions (InAs/Nb) using a three-dimensional discrete lattice model described by the Bogoliubov–de Gennes Hamiltonian in the tight-binding approximation, and compute the Andreev bound state spectrum and current-phase relations. Recent experiments [Zuo et al., Phys. Rev. Lett. 119, 187704 (2017) and Gharavi et al., arXiv:1405.7455] reveal critical current oscillations in these devices, and our simulations confirm these to be an interference effect of the transverse subbands in the nanowire. We add disorder to model coherent scattering and study its effect on the critical current oscillations, with an aim to gain a thorough understanding of the experiments. The oscillations in the disordered junction are highly sensitive to the particular realization of the random disorder potential, and to the gate voltage. A macroscopic current measurement thus gives us information about the microscopic profile of the junction. Finally, we study dephasing in the channel by including elastic phase-breaking interactions. The oscillations thus obtained are in good qualitative agreement with the experimental data, and this signifies the essential role of phase-breaking processes in III-V semiconductor nanowire Josephson junctions.

We propose a microscopic magnetoelectric model in which the coupling between spins and electric dipoles is mediated by lattice distortions. The magnetic sector is described by a spin $S=1/2$ Heisenberg model coupled directly to the lattice via a standard spin-Peierls term and indirectly to the electric dipole variables via the distortion of the surrounding electronic clouds. Electric dipoles are described by Ising variables for simplicity. We show that the effective magnetoelectric coupling which arises due to the interconnecting lattice deformations is quite efficient in one-dimensional arrays. More precisely, we show using bosonization and extensive density matrix renormalization group numerical simulations that increasing the magnetic field above the spin-Peierls gap, a massive polarization switch-off occurs due to the proliferation of soliton pairs. We also analyze the effect of an external electric field when the magnetic system is in a gapped (plateau) phase and show that the magnetization can be electrically switched between clearly distinct values. More general quasi-one-dimensional models and two-dimensional systems are also discussed.

We study the electronic structure, magnetic state, and phase stability of paramagnetic ${\mathrm{BiNiO}}_3$ near a pressure-induced Mott insulator-to-metal transition (MIT) by employing a combination of density functional and dynamical mean-field theory. We obtain that ${\mathrm{BiNiO}}_3$ exhibits an anomalous negative-charge-transfer insulating state, characterized by charge disproportionation of the Bi $6s$ states, with ${\mathrm{Ni}}^{2+}$ ions. Upon a compression of the lattice volume by $\sim 4.8$%, ${\mathrm{BiNiO}}_3$ is found to make a Mott MIT, accompanied by the change of crystal structure from triclinic $P\overline{1}$ to orthorhombic $Pbnm$. The pressure-induced MIT is associated with the melting of charge disproportionation of the Bi ions, caused by a charge transfer between the Bi $6s$ and O $2p$ states. The Ni sites remain to be ${\mathrm{Ni}}^{2+}$ across the MIT, which is incompatible with the valence-skipping ${\mathrm{Ni}}^{2+}$/${\mathrm{Ni}}^{3+}$ model. Our results suggest that the pressure-induced change of the crystal structure drives the MIT in ${\mathrm{BiNiO}}_3$.

We present a detailed study of the electronic and spin-orbit properties of single and bilayer graphene in proximity to the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Our approach is based on first-principles calculations, combined with symmetry derived model Hamiltonians that capture the low-energy band properties. We consider single and bilayer graphene on 1–3 quintuple layers of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and extract orbital and proximity induced spin-orbit coupling (SOC) parameters. We find that graphene gets significantly hole doped (350 meV), but the linear dispersion is preserved. The proximity induced SOC parameters are about 1 meV in magnitude, and are of valley-Zeeman type. The induced SOC depends weakly on the number of quintuple layers of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. We also study the effect of a transverse electric field that is applied across heterostructures of single and bilayer graphene above 1 quintuple layer of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Our results show that band offsets, as well as proximity induced SOC parameters, can be tuned by the field. Most interesting is the case of bilayer graphene, in which the band gap, originating from the intrinsic dipole of the heterostructure, can be closed and reopened again, with inverted band character. The switching of the strong proximity SOC from the conduction to the valence band realizes a spin-orbit valve. Additionally, we find a giant increase of the proximity induced SOC of about 200% when we decrease the interlayer distance between graphene and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ by only 10%. Finally, for a different substrate material ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$, band offsets are significantly different, with the graphene Dirac point located at the Fermi level, while the induced SOC strength stays similar in magnitude compared to the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ substrate.

Frustration in quantum spin systems promote a variety of novel quantum phases. An important example is the frustrated spin-1 model on the square lattice with the nearest-neighbor bilinear ($J_1$) and biquadratic ($K_1$) interactions. We provide strong evidence for a nematic spin liquid phase in a range of $K_1/J_1$ near the SU(3)-symmetric point ($J_1=K_1$), based on the linear flavor-wave theory and extensive density matrix renormalization group calculation. This phase displays no spin dipolar or quadrupolar order, preserves translational symmetry but spontaneously breaks $C_4$ lattice rotational symmetry, and possesses fluctuations peaked at the wave vector $(\pi ,2\pi /3)$. The spin excitation gap drops rapidly with system size and appears to be gapless, and the nematic order is attributed to the dominant $(\pi ,2\pi /3)$ fluctuations. Our results provide a novel mechanism for electronic nematic order and, more generally, open up a new avenue to explore frustration-induced exotic ground states.

Magnetic skyrmions, topologically stable spin swirling objects, have attracted a great interest due to their potential applications in future spintronics and ultrahigh dense magnetic memory devices. Existence of a skyrmion phase as well as first- and second-order phase transitions make ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ a promising candidate for investigating complex magnetic phenomena. Here, we report that both first- and second-order magnetic phase transitions are responsible in determining the phase diagram with at least two multicritical points in ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$. A fluctuation-induced first-order transition is realized as a precursor for the skyrmion phase over a small window of temperature and magnetic field. The evolution of isothermal entropy at the phase transition provides evidence for a tricritical point. Furthermore, the existence of commensurate and incommensurate phases along with the coexistence of three second-order phase transitions provide evidence for the existence of a Lifshitz point.

We propose a modified Boltzmann nonlinear electric-transport framework which differs from the nonlinear generalization of the linear Boltzmann formalism by a contribution that has no counterpart in linear response. This contribution follows from the interband-coherence effect of dc electric fields during scattering and is related to the interband Berry connection. As an application, we demonstrate it in the second-order nonlinear Hall effect of the tilted massive Dirac model. The intuitive Boltzmann constructions are confirmed by a quantum kinetic theory, which shows that the arbitrary $n\mathrm{th}$-order nonlinear dc response up to the first three leading contributions in the weak disorder potential is handled by the same few gauge-invariant semiclassical ingredients.

We investigate spatiotemporal chaos dynamics in a finite nanoparticles array with Kerr-type nonlinear response, excited by an incident plane wave of varying intensity and tunable frequency close to the localized plasmon resonance of a single particle. Considering dipole-dipole coupling between the nanoparticles described by their polarizability, we compute the temporal evolution of the dipoles and numerically extract the Lyapunov spectra, allowing us to characterize different dynamical behaviors. Furthermore, we estimate the Kaplan-Yorke dimension that provides a measure of the strange attractor complexity. We show that time-modulated solutions which are generated at the onset of modulational instability experience secondary instabilities leading to a complex nonlinear dynamic. It is also shown that in the highly nonlinear regime, the spatiotemporal chaos is robust and exists in a large range of parameters that we have determined numerically.

A comprehensive analysis of hybrid TM-TE polarized surface electromagnetic waves supported by different few-layer anisotropic metasurfaces is presented. A generalized $4\times 4$ T-matrix formalism for arbitrary anisotropic two-dimensional (2D) layers is developed, from which the general relations for the surface waves dispersions and scattering coefficients are deduced. Using this formalism and the effective conductivity approach, the dispersions and iso-frequency contours (IFCs) topology of the surface waves in various hybrid uniaxial metasurfaces are studied. The existence of hyperbolic plasmon-exciton polaritons in plasmon-exciton hybrids and hyperbolic acoustic waves with strong confinement in both out-of-plane and in-plane directions in uniaxial plasmonic bilayers are predicted. In plasmonic uniaxial metasurfaces on metal films, the elliptic and hyperbolic backward surface waves with negative group velocity are predicted and additional topological transitions in both elliptic and hyperbolic IFCs of the hybrid surface waves are revealed. Ultrathin twisted uniaxial plasmonic bilayers are proposed as systems with the IFCs topological transitions highly sensitive to the layers twist. The developed formalism may become a useful tool in the calculation of multifunctional few-layer metasurfaces or van der Waals heterostructures based on 2D materials with in-plane anisotropy, where the TM-TE polarization mixing must be considered. The predicted effects may open new horizons in the development and applications of planar optical technologies.

Vertical stacking of ultrathin two-dimensional materials via weak van der Waals (vdW) interactions is identified as an important technique for tuning the physical properties and designing viable products for nanoelectronics, spintronics, and renewable energy source applications. The geometry, electronic, and photocatalytic properties of vdW heterostructures of GeC and Janus transition metal dichalcogenides $M\mathrm{SSe}$ ($M$ = Mo, W) monolayers are investigated by performing first-principles calculations. Two different possible models of GeC-$M\mathrm{SSe}$ heterostructures are presented with an alternative order of chalcogen atoms at opposite surfaces in $M\mathrm{SSe}$. The most favorable stacking pattern of both models is dynamically and energetically feasible. A direct type-II band alignment is obtained in both models of understudy heterobilayer systems. The spin orbit coupling (SOC) effect causes considerable Rashba spin splitting in both $M\mathrm{SSe}$ monolayers. In particular, a greater Rashba spin polarization is demonstrated in model 1 (GeC-WSSe) than model 2 (GeC-MoSSe) caused by the alternative order of chalcogen atoms and larger SOC effect of heavier W than Mo atoms, which provides a platform for experimental and theoretical understanding of designing two-dimensional spintronic devices. More interestingly, the appropriate band alignments of model 1 with the standard water redox potentials enable its capability to dissociate water into ${\text{H}}^{+}/{\text{H}}_2$ and ${\text{O}}_2/{\text{H}}_2\text{O}$. In contrast to model 1, model 2 can only oxidize water into ${\text{O}}_2/{\text{H}}_2\text{O}$. The simulated design of GeC-$M\mathrm{SSe}$ is predicted for promising use in future electronic, spintronics, and photocatalytic water splitting.

We generalize the notion of a dissipationless, topological Hall viscosity tensor to optical phonons in thin-film Weyl semimetals. By using the strained porphyrin thin-film Weyl semimetal as a model example, we show how optical phonons can couple to Weyl electrons as chiral pseudogauge fields. These chiral vector fields lead to a novel dissipationless two-rank viscosity tensor in the effective dynamics of optical phonons whose origin is the chiral anomaly. We also compute the contribution to this two-rank Hall viscosity tensor due to the presence of an external magnetic field whose origin is the conventional Hall response of Weyl electrons. Finally, the phonon dispersion relations of the system at the long-wavelength limit with and without an electromagnetic field are calculated, showing a measurable shift in the Raman response of the system. Our results can be investigated by Raman scattering or infrared spectroscopy by attenuated total reflectance experiments.

${}^{51}\mathrm{V}$ nuclear magnetic resonance (NMR) and magnetization studies on ${\mathrm{CeTi}}_{1-x}{\mathrm{V}}_x{\mathrm{Ge}}_3$ have been performed to explore the evolution from the ferromagnetic ($x=0.113$) to the antiferromagnetic Kondo lattice state ($x=1$), with focus on the emergence of a possible ferromagnetic quantum critical point (FMQCP) at $x_c\approx 0.4$. From the temperature dependence of the nuclear spin-lattice relaxation rate, $1/T_{1}T$, and the Knight shift, $K$, for $x=0.113$ and $x=1$ a considerable competition between ferro- and antiferromagnetic correlations is found. Around the critical concentration ($x=0.35,0.405$), quantum-critical spin fluctuations entail weak antiferromagnetic spin fluctuations admixed with ferromagnetic spin fluctuations. The FMQCP in ${\mathrm{CeTi}}_{1-x}{\mathrm{V}}_x{\mathrm{Ge}}_3$ therefore is not purely ferromagnetic in nature.

Superconductor-ferromagnet hybrid structures (SF) have attracted much interest in the past decades, due to a variety of interesting phenomena predicted and observed in these structures. One of them is the so-called inverse proximity effect. It is described by a spin polarization of Cooper pairs, which occurs not only in the ferromagnet (F), but also in the superconductor (S), yielding a finite magnetic moment $M_{\text{S}}$ inside the superconductor. This effect has been predicted and experimentally studied. However, interpretation of the experimental data is mostly ambiguous. Here, we study theoretically the impact of the spin polarized Cooper pairs on the Josephson effect in an SFS junction. We show that the induced magnetic moment $M_{\text{S}}$ does depend on the phase difference $\phi$ and, therefore, will oscillate in time with the Josephson frequency $2eV/\hslash$ if the current exceeds a critical value. Most importantly, the spin polarization in the superconductor causes a significant change in the Fraunhofer pattern, which can be easily accessed experimentally.

Reentrant melting (in which a substance's melting point starts to decrease beyond a certain pressure) is believed to be an unusual phenomenon. Among the elements, it has so far only been observed in a very limited number of species, e.g., the alkali metals. Our density functional theory calculations reveal that this behavior actually extends beyond alkali metals to include magnesium, which also undergoes reentrant melting, though at the much higher pressure of $\sim 300$ GPa. We find that the origin of reentrant melting is the faster softening of interatomic interactions in the liquid phase than in the solid, as pressure rises. We propose a simple approach to estimate pressure-volume relations and show that this characteristic softening pattern is widely observed in metallic elements. We verify this prediction in the case of aluminum by finding reentrant melting at $\sim 4000$ GPa. These results suggest that reentrant melting may be a more universal feature than previously thought.