As a potential system for realizing strong spin-orbit coupling (SOC), $5d\text{−}A\mathrm{Ta}{\mathrm{O}}_3$ perovskite oxides have recently received extensive attention. This study presents ${\mathrm{SrTaO}}_3/\mathrm{KTa}{\mathrm{O}}_3 5d\text{−}5d$ heterostructures with thickness-induced crossover from quasi-two-dimensional (2D) to three-dimensional (3D) transport behaviors. The sheet carrier concentration and carrier mobility of the system depend on the ${\mathrm{SrTaO}}_3$ film thickness. The highest mobility of $430\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ is observed for the heterostructure with a 16-nm-thick ${\mathrm{SrTaO}}_3$ film, which comprises coexisting 2D and 3D transport components. Weak antilocalization, anisotropic magnetoresistance, and the plane Hall effect are observed, which robustly confirm the existence of strong SOC in the heterostructure. The tunable transport and strong correlation effect make the ${\mathrm{SrTaO}}_3/\mathrm{KTa}{\mathrm{O}}_3$ heterostructure not only a suitable platform for investigating basic physical phenomena but also a potential material candidate for future spin devices.

We report the electronic structures and transport properties of a chiral crystal ${\mathrm{NbGe}}_2$, which is a candidate for a coupled electron-phonon liquid. The electrical resistivity and thermoelectric power of ${\mathrm{NbGe}}_2$ exhibit clear differences compared to those of ${\mathrm{NbSi}}_2$ even though both niobium ditetrelides are isostructural and isoelectronic. We discuss the intriguing transport properties of ${\mathrm{NbGe}}_2$ based on a van Hove-type singularity in the density of states. The analysis of de Haas-van Alphen oscillations measured by the field modulation and magnetic torque methods reveals the detailed shape of the Fermi surface of ${\mathrm{NbGe}}_2$ by comparison with the results of energy band structure calculations using a local density approximation. The electron and hole Fermi surfaces of ${\mathrm{NbGe}}_2$ split into two because of the antisymmetric spin-orbit interaction. The temperature dependence of quantum oscillations indicates that the effective mass is isotropically enhanced in ${\mathrm{NbGe}}_2$ due to strong electron-phonon interaction.

We consider an isotropic spin-degenerate interacting uniform $D$-dimensional electron gas (DDEG) with $D>1$ within the Luttinger-Ward (LW) formalism. We derive the asymptotically exact semiclassical/infrared limit of the LW functional at large distances, $r\gg {\lambda }_F$, and large times, $\tau \gg 1/E_F$, where ${\lambda }_F$ and $E_F$ are the Fermi wavelength and the Fermi energy, respectively. The LW functional is represented by skeleton diagrams, each skeleton diagram consists of appropriately connected dressed fermion loops. First, we prove that every $D$-dimensional skeleton diagram consisting of a single fermion loop is reduced to a one-dimensional (1D) fermion loop with the same diagrammatic structure, which justifies the name dimensional reduction. This statement, combined with the fermion loop cancellation theorem (FLCT), agrees with results of multidimensional bosonization. Here we show that the backscattering and the spectral curvature, both explicitly violate the FLCT and both are irrelevant for a 1DEG, become relevant at $D>1$ and $D>2$, respectively. The reason for this is a strong infrared divergence of the skeleton diagrams containing multiple fermion loops at $D>1$. These diagrams, which are omitted within the multidimensional bosonization approaches, account for the noncollinear scattering processes. Thus, the dimensional reduction provides the framework to go beyond predictions of the multidimensional bosonization. A simple diagrammatic structure of the reduced LW functional is another advantage of our approach. The dimensional reduction technique is also applicable to the thermodynamic potential and various approximations, from perturbation theory to self-consistent approaches.

The Callan-Harvey mechanism in the $2+1$-dimensional ($2+1$-D) Jackiw-Rebbi model is revisited. We analyzed Callan-Harvey anomaly inflow in the massive Chern insulator (quantum anomalous Hall system) subject to an external electric field. In addition to the conventional current flowing from the bulk to the edge due to parity anomaly, we considered the dissipation of the edge charge due to the interaction with an external bosonic bath in $2+1$ D and due to an external bath of photons in $3+1$ D. In the case of a $2+1$-D bosonic bath, we found the new stationary state, which is defined by the balance between the Callan-Harvey current and the outgoing flow caused by the dissipation processes. In the case a of $3+1$-D photon bath, we found a critical electric field below which this balance state can be achieved, but above which there is no such balance. However, if one considers a more realistic model with a momentum-dependent mass $\beta p^2$, the edge state merges into the bulk when the momentum is large, leading the system to the balance. Furthermore, we estimated the photon-mediated transition rate between a $2+1$-D bulk and $1+1$-D topological edge state of the order of $1 {\mathrm{ns}}^{-1}$ at room temperature.

We study theoretically the electronic properties of the artificial quantum dot honeycomb lattice defined in a two-dimensional electron gas, focusing on the possibility of achieving a regime in which electronic correlations play a dominant role. At first, we establish a noninteracting model compatible with recently studied experimentally devices. According to the values of the obtained electron-electron interaction integrals, we postulate that the inclusion of inherent electron-gas self-screening is indispensable to reconstruct the experimental observations. Applying the Thomas-Fermi type of screening, we show that the radius of the antidot is crucial to achieve a correlated state in which phenomena like antiferromagnetic ordering and interaction-induced insulating state appear. We estimate the conditions for which the electronically correlated state in an artificial honeycomb lattice can be realized.

It has been demonstrated that small plaquettes of quantum dot spin qubits are capable of simulating condensed matter phenomena that arise from the Hubbard model, such as the collective Coulomb blockade and Nagaoka ferromagnetism. Motivated by recent materials developments, we investigate a bilayer arrangement of quantum dots with four dots in each layer which exhibits a complex ground state behavior. We find using a generalized Hubbard model with long-range Coulomb interactions, several distinct magnetic configurations occur as the Coulomb interaction strength is varied, with possible ground states that are ferromagnetic, antiferromagnetic, or having both one antiferromagnetic and one ferromagnetic layer. We map out the full phase diagram of the system as it depends on the inter- and intralayer Coulomb interaction strengths, and find that for a single layer, a similar but simpler effect occurs. We also predict interesting contrasts among electron, hole, and electron-hole bilayer systems arising from complex correlation physics. Observing the predicted magnetic configuration in already-existing few-dot semiconductor bilayer structures could prove to be an important assessment of current experimental quantum dot devices, particularly in the context of spin-qubit-based analog quantum simulations.

The exploration of unusual Weyl phonons has long been regarded as one of the central topics in topological physics. Our symmetry analysis shows that symmetry-enforced type-II Weyl phonons may exist under the protections from two mirror symmetries (${\mathcal{M}}_x$ and ${\mathcal{M}}_y$), a twofold rotation symmetry ($C_{2z}$) and time-reversal symmetry ($\mathcal{T}$), and meanwhile, another kind of unusual Weyl phonon, i.e., hybrid Weyl nodal-line phonons, may be constructed on a square and four straight Weyl nodal lines. Our first-principles calculations demonstrate that these two unusual Weyl phonons may coexist in a real material $P\overline{4}m2$-carbon. More importantly, another kind of nontrivial surface states, i.e., which we refer to as hybrid surface states, are produced both by type-II Weyl phonons and by the hybrid Weyl nodal-line phonons. Our work provides an ideal material platform to study type-II Weyl phonons and hybrid Weyl nodal-line phonons in phononic systems.

Generating photogalvanic effects in centrosymmetric materials can provide new opportunities for developing passive photodetectors and energy-harvesting devices. In this work, we investigate the photogalvanic effects in centrosymmetric two-dimensional materials, AA- and AB-stacked bilayer graphene, by applying an external gate voltage to break the inversion symmetry. Using a tight-binding model to describe the electronic states, the injection coefficients for circular photogalvanic effects and shift conductivities for linear photogalvanic effects are calculated for both materials with light wavelengths ranging from terahertz to visible. We find that gate voltage induced photogalvanic effects can be very significant for AB-stacked bilayer graphene, generating a maximal dc current on the order of milliamperes for a 1 $\mu \mathrm{m}$ wide sample illuminated by a light intensity of $0.1\mathrm{GW}/{\mathrm{cm}}^2$, which is determined by the optical transition around the band gap and van Hove singularity points. Although such effects in AA-stacked bilayer graphene are about two orders of magnitude smaller than those in AB-stacked bilayer graphene, the spectrum is interestingly limited in a very narrow photon energy window, which is associated with the interlayer coupling strength. A detailed analysis of the light polarization dependence is also performed. The gate voltage and chemical potential can be used to effectively control the photogalvanic effects.

Topological insulators are an important family of quantum materials that exhibit a Dirac point (DP) in the surface band structure but have a finite band gap in bulk. A large degree of spin-orbit interaction and low band gap is a prerequisite for stabilizing DPs on selective atomically flat cleavage planes. Tuning of the DP in these materials has been suggested via modifications to the atomic structure of the entire system. Using the example of ${\mathrm{As}}_2{\mathrm{Te}}_3$ and ${\mathrm{ZnTe}}_5$, which are not TIs, we show that a quantum phase transition can be induced in atomically flat and stepped surfaces, for ${\mathrm{As}}_2{\mathrm{Te}}_3$ and ${\mathrm{ZrTe}}_5$, respectively. This is achieved by establishing a framework for controlling electronic properties that is focused on local perturbations at key locations that we call substructural elements. We exemplify this framework through a unique method of isovalent sublayer anion doping and biaxial strain.

An uncertainty in studying twisted bilayer graphene (TBG) is the minimum-energy geometry, which strongly affects the electronic structure. The minimum-energy geometry is determined by the potential-energy surface, which is dominated by van der Waals (vdW) interactions. In this paper, large-scale diffusion quantum Monte Carlo (QMC) simulations are performed to evaluate the energy of bilayer graphene at various interlayer distances for four stacking registries. An accurate registry-dependent potential is fit to the QMC data and is used to describe interlayer interactions in the geometry of near-magic-angle TBG. The band structure for the optimized geometry is evaluated using the accurate local-environment tight-binding model. We find that compared to QMC, DFT-based vdW interactions can result in errors in the corrugation magnitude by a factor of 2 or more near the magic angle. The error in corrugation then propagates to the flat bands in twisted bilayer graphene, where the error in corrugation can affect the bandwidth by about 30% and can change the nature and degeneracy of the flat bands.

The phenomenon of spin-dependent quantum scattering in two-dimensional (2D) pseudospin-1/2 Dirac materials leading to a relativistic quantum chimera was recently uncovered. We investigate spin-dependent Dirac electron optics in 2D pseudospin-1 Dirac materials, where the energy-band structure consists of a pair of Dirac cones and a flat band. In particular, with a suitable combination of external electric fields and a magnetic exchange field, electrons with a specific spin orientation (e.g., spin-down) can be trapped in a class of long-lived edge modes, generating resonant scattering. The spin-dependent edge states are a unique feature of flat-band Dirac materials and have no classical correspondence. However, electrons with the opposite spin (i.e., spin up) undergo conventional quantum scattering with a classical correspondence, which can be understood in the framework of Dirac electron optics. A consequence is that the spin-down electrons produce a large scattering probability with broad scattering angle distribution in both near- and far-field regions, while the spin-up electrons display the opposite behavior. Such characteristically different behaviors of the electrons with opposite spins lead to spin polarization that can be as high as nearly 100%.

We investigate the orbital Hall effect through a mesoscopic device with momentum-space orbital texture that is connected to four semi-infinite terminals embedded in the Landauer-Büttiker configuration for quantum transport. We present analytical and numerical evidence that the orbital Hall current exhibits mesoscopic fluctuations, which can be interpreted in the framework of random matrix theory (RMT) (as with spin Hall current fluctuations). The mesoscopic fluctuations of orbital Hall current display two different amplitudes of 0.36 and 0.18 for weak and strong spin-orbit coupling, respectively. The amplitudes are obtained by analytical calculation via RMT and are supported by numerical calculations based on the tight-binding model. Furthermore, the orbital Hall current fluctuations lead to two relationships between the orbital Hall angle and conductivity. Finally, we confront the two relations with experimental data of the orbital Hall angle, which shows good concordance between theory and experiment.

Emergent Dirac fermions provide the starting point for understanding the plethora of novel condensed matter phases. The nature of the associated phases and phase transitions crucially depends on both the emergent symmetries as well as the implementation of the microscopic ones on the low-energy Dirac fermions. Here, we show that $j=3/2$ electrons in spin-orbit coupled materials on honeycomb lattice can give rise to SU(8) symmetric Dirac semimetals with symmetry implementation very different from that of graphene. This nontrivial embedding of the microscopic symmetries in the low energy is reflected in the nature of phases proximate to the Dirac semimetal. Such phases can arise from finite short-range electron-electron interactions. In particular, we identify 24 such phases—divided into three classes—and their low-energy properties obtained by condensing particle-number conserving fermion bilinears that break very different microscopic symmetries and/or are topologically protected by symmetries. The latter includes interesting generalizations of quantum spin-Hall phases. Remarkably some of the resultant phases still support a subset of gapless fermions—protected by a subgroup of SU(8)—resulting in interesting density wave semimetals. Near the phase transitions to such density wave semimetals, the surviving gapless fermions strongly interact with the bosonic order parameter field and give rise to novel quantum critical points. Our study is applicable to a wide class of $d^1$ and $d^3$ transition metals with strong spin-orbit coupling and predicts that such materials can harbor a very rich interplay of symmetries and competing interactions in the intermediate correlation regime.

Using rigorous ab initio calculations of many-body electron-electron, electron-phonon, electron-hole, and exciton-phonon couplings, we demonstrate that the recently discovered semiconducting phase, clustered-P1 borophene, can exhibit an optical gap of 0.74 eV and possesses a substantial binding energy of 1.5 eV at room temperature. Additionally, it features a significantly faster nonradiative recombination rate at room temperature. The profound impact of dynamical self-energies originating from pure electron-phonon interactions results in a noteworthy zero-point renormalization of the quasiparticle direct and indirect gaps, leading to large shifts of 73 and 91 meV, respectively. We demonstrate strong, incoherent interactions between electrons and holes with lattice vibrations, resulting in a significant redshifting of the absorption spectrum at 0 K. The pivotal phonon modes responsible for shaping the lowest bound exciton originates from the spectral range of 350–950 ${\mathrm{cm}}^{-1}$ and also exhibits resilience against temperature variations. Notably, the room temperature nonradiative lifetime of the lowest bound exciton is found to be significantly smaller ($\sim 40$ fs) compared to its intrinsic radiative lifetime ($\sim 0.6$ ns). These remarkable outcomes collectively suggest that clustered-P1 borophene is a formidable contender for the development of near-ultraviolet optoelectronic devices.

Domain wall morphologies in ferroelectrics are believed to be largely shaped by electrostatic forces. Here, we show that for conducting domain walls, the morphology also depends on the details of the charge-carrier band structure. For concreteness, we focus on transition-metal perovskites like ${\mathrm{BaTiO}}_3$ and ${\mathrm{SrTiO}}_3$. These have a triplet of $t_{2g}$ orbitals attached to the Ti atoms that form the conduction bands when electron doped. We solve a set of coupled equations—Landau-Ginzburg-Devonshire (LGD) equations for the polarization, tight-binding Schrödinger equations for the electron bands, and Gauss' law for the electric potential—to obtain polarization and electron density profiles as a function of electron density. We find that at low electron densities, the electron gas is pinned to the surfaces of the ferroelectric by a Kittel-like domain structure. As the electron density increases, the domain wall evolves smoothly through a zigzag head-to-head structure, eventually becoming a flat head-to-head domain wall at high density. We find that the Kittel-like morphology is protected by orbital asymmetry at low electron densities, while at large electron densities the high density of states of the multiorbital band structure provides effective screening of depolarizing fields and flattens the domain wall relative to single-orbital models. Finally, we show that in the zigzag phase, the electron gas develops tails that extend away from the domain wall, in contrast to naïve expectations.

We revisit the Hofstadter butterfly for a subset of topologically trivial Bloch bands arising from a continuum free electron Hamiltonian in a periodic lattice potential. We employ the recently developed procedure, which was previously used to analyze the case of topologically nontrivial bands [Wang and Vafek, Phys. Rev. B 106, L121111 (2022)], to construct the finite-field Hilbert space from the zero-field hybrid Wannier basis states. Such states are Bloch extended along one direction and exponentially localized along the other. The method is illustrated for square and triangular lattice potentials and is shown to reproduce all the main features of the Hofstadter spectrum obtained from a numerically exact Landau level expansion method. In the regime where magnetic length is much longer than the spatial extent of the hybrid Wannier state in the localized direction we recover the well-known Harper equation. Because the method applies to both topologically trivial and nontrivial bands, it provides an alternative and efficient approach to moiré materials in magnetic field.

The widely-used linear-response (LR) theory of thermal conduction in the quantum regime rests on the yet unproven assumption that the thermal conductivity is invariant with respect to the gauge of the energy density of the system. This assumption manifests itself clearly in, e.g., Hardy's formulation of the heat-flux operator [Hardy, Phys. Rev. 132, 168 (1963)]. In this paper, we rigorously prove this assumption. Our proof, being valid for the nuclear and electronic subsystems, not only puts the state-of-the-art theory on solid grounds, but also enables going beyond the scope of the widely-used Boltzmann transport equation (BTE) within the LR framework for many-body systems.

Hole spin qubits in group-IV semiconductors, especially Ge and Si, are actively investigated as platforms for ultrafast electrical spin manipulation thanks to their strong spin-orbit coupling. Nevertheless, the theoretical understanding of spin dynamics in these systems is in the early stages of development, particularly for in-plane magnetic fields as used in the vast majority of experiments. In this work, we present a comprehensive theory of spin physics in planar Ge hole quantum dots in an in-plane magnetic field, where the orbital terms play a dominant role in qubit physics, and provide a brief comparison with experimental measurements of the angular dependence of electrically driven spin resonance. We focus the theoretical analysis on electrical spin operation, phonon-induced relaxation, and the existence of coherence sweet spots. We find that the choice of magnetic field orientation makes a substantial difference for the properties of hole spin qubits. Specifically, we find that (i) EDSR for in-plane magnetic fields varies nonlinearly with the field strength and weaker than for perpendicular magnetic fields. (ii) The EDSR Rabi frequency is maximized when the a.c. electric field is aligned parallel to the magnetic field, and vanishes when the two are perpendicular. (iii) The orbital magnetic field terms make the in-plane $g$-factor strongly anisotropic in a squeezed dot, in excellent agreement with experimental measurements. (iv) Focusing on random telegraph noise, we show that the effect of noise in an in-plane magnetic field cannot be fully mitigated, as the orbital magnetic field terms expose the qubit to all components of the defect electric field. These findings will provide a guideline for experiments to design ultrafast, highly coherent hole spin qubits in Ge.