We studied neutral excitations in a two-dimensional electron gas with an orbital momentum $\mathrm{\Delta }M=1$ and spin projection over the magnetic field axis $\mathrm{\Delta }{S}_z=1$ in the vicinity of a filling factor of 3/2. It is shown that the 3/2 state is a singular point in the filling factor dependence of the spin ordering of the two-dimensional electron gas. In the vicinity of $\nu =3/2$, a significant increase in the relaxation time ($\tau =13\mu \text{s}$) for the excitations to the ground state is exhibited even though the number of vacancies in the lowest energy level is macroscopically large. The decrease in the relaxation rate is related to the spin texture transformation in the ground state induced by spin flips and electron density rearrangement. Based on the experimental data we believe that the 3/2 state is an example of locally incompressible fractional quantum Hall state.

The charge density wave (CDW) is a unique phenomenon mostly realized in two-dimensional (2D) metallic layered transition metal dichalcogenides. We report on the observation of commensurate CDW (C-CDW) and incommensurate (I-CDW) transition in single crystal $1\mathrm{T}\text{−}\mathrm{V}{\mathrm{Se}}_2$ using the spectroscopy technique in the temperature range of 50–120 K. The room temperature Raman spectra showed a sharp $A_{1g}$ mode $\sim 206\mathrm{c}{\mathrm{m}}^{-1}$ along with two additional modes associated to $E_g\sim 257\mathrm{c}{\mathrm{m}}^{-1}$ symmetry and two-phonon $\left(2_{ph}\right)\sim 332\mathrm{c}{\mathrm{m}}^{-1}$ interactions. The onset temperature of I-CDW and C-CDW is estimated from resistance measurements supported by magnetic measurements. Remarkably, at the onset of I-CDW ∼ 115 K, a significant enhancement in the intensity of the weak $E_g$ mode is observed along with emergence of a doubly degenerate $E_g\left(2\right)$ mode $\sim 144\mathrm{c}{\mathrm{m}}^{-1}$, while below 70 K, the emergence of the $A_{1g}$ mode $\sim 170\mathrm{c}{\mathrm{m}}^{-1}$ signifies the onset of C-CDW. The observation of a sudden rise in the intensity of the $E_g$ mode $\sim 257\mathrm{c}{\mathrm{m}}^{-1}$ mode, below I-CDW, showed the involvement of electron-phonon coupling in $1\mathrm{T}\text{−}\mathrm{V}{\mathrm{Se}}_2$.

$\alpha \text{−}\mathrm{GeTe}\left(111\right)$ is a noncentrosymmetric ferroelectric material for which a strong spin-orbit interaction gives rise to giant Rashba split states in the bulk and at the surface. The detailed dispersions of the surface states inside the bulk band gap remains an open question because they are located in the unoccupied part of the electronic structure, making them inaccessible to static angle-resolved photoemission spectroscopy. We show that this difficulty can be overcome via in situ potassium doping of the surface, leading to a rigid shift of 80 meV of the surface states into the occupied states. Thus, we resolve, in great detail, their dispersion and highlight their crossing at the $\overline{\mathrm{\Gamma }}$ point, which, in comparison with density functional theory calculations, definitively confirms the Rashba mechanism.

We consider the quantum first detection problem for a particle evolving on a graph under repeated projective measurements with fixed rate $1/\tau$. A general formula for the mean first detected transition time is obtained for a quantum walk in a finite-dimensional Hilbert space where the initial state $|{\psi }_{\mathrm{in}}\rangle$ of the walker is orthogonal to the detected state $|{\psi }_{\mathrm{d}}\rangle$. We focus on diverging mean transition times, where the total detection probability exhibits a discontinuous drop of its value by mapping the problem onto a theory of fields of classical charges located on the unit disk. Close to the critical parameters of the model, we find simple expressions describing the blow-up of the mean transition time. Using previous results on the fluctuations of the return time, corresponding to $|{\psi }_{\mathrm{in}}\rangle =|{\psi }_{\mathrm{d}}\rangle$, we find close to these critical parameters that the mean transition time is proportional to the fluctuations of the return time, an expression reminiscent of the Einstein relation.

The anomalous Hall effect is commonly considered a signature of ferromagnetism. However, recently, an enormous anomalous Hall conductivity was measured in the compensated kagome magnets ${\mathrm{Mn}}_3\mathrm{Sn}$ and ${\mathrm{Mn}}_3\mathrm{Ge}$. The occurrence of this effect is allowed by the magnetic point group of these materials; however, its emergence is still lacking a microscopic explanation. Herein we show that the spin-orbit coupling and an out-of-plane tilting of the texture are equivalent for several kagome magnets. Consequently, a coplanar system with spin-orbit coupling behaves as if it were virtually noncoplanar. We show via tight-binding model calculations that the Hall effect can mainly be interpreted as a topological Hall effect generated by the opening angle of the virtually tilted texture. Furthermore, upon tilting the fixed texture out of the kagome plane, we find a critical tilting angle for which the Hall conductivity vanishes for all energies. In this case, the Hamiltonian is invariant under a combined time-reversal and mirror symmetry, because the virtual texture is coplanar.

We numerically compute the density of states (DOS) of interacting disordered zigzag graphene nanoribbon (ZGNR) having midgap states showing $e/2$ fractional edge charges. The computed Hartree-Fock DOS is linear at the critical disorder strength where the gap vanishes. This implies an $I\text{−}V$ curve of $I\propto V^2$. Thus, $I\text{−}V$ curve measurement may yield evidence of fractional charges in interacting disordered ZGNR. We show that even a weak disorder potential acts as a singular perturbation on zigzag edge electronic states, producing drastic changes in the energy spectrum. Spin-charge separation and fractional charges play a key role in the reconstruction of edge antiferromagnetism. Our results show that an interacting disordered ZGNR is a topologically ordered Mott-Anderson insulator.

The favorable combination of Raman spectroscopy of the lattice vibrations with ab initio phonon calculations enables us to pinpoint a low-temperature orthorhombic phase which hosts the cryptic 1 K superconducting phase of the $5d$ pyrochlore-type superconductor ${\mathrm{Cd}}_2{\mathrm{Re}}_2{\mathrm{O}}_7$. Raman studies are performed on crystals with the natural isotope abundance and cadmium and oxygen isotope enriched samples. Characteristic splitting and the appearance of sharp phonon modes in the temperature-dependent Raman spectra prove a structural phase below $\sim 80$ K, induced by a soft-mode phonon instability of the intermediate-temperature tetragonal crystal structure. The theory predicts that this low-temperature orthorhombic crystal structure is described by the space group $F222$; it exhibits no phonon instabilities and has the lowest total energy. Future experiments on the $F222$ phase should establish whether spin-triplet components contribute to the superconducting phase.

Strontium ruthenate (${\mathrm{Sr}}_2{\mathrm{RuO}}_4$) has long been thought to host a spin-triplet chiral $p$-wave superconducting state. However, the singletlike response observed in recent spin-susceptibility measurements casts serious doubts on this pairing state. Together with the evidence for broken time-reversal symmetry and a jump in the shear modulus $c_{66}$ at the superconducting transition temperature, the available experiments point towards an even-parity chiral superconductor with $k_z(k_x\pm i{k}_y)$-like $E_g$ symmetry, which has consistently been dismissed based on the quasi-two-dimensional electronic structure of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. Here, we show how the orbital degree of freedom can encode the two-component nature of the $E_g$ order parameter, allowing for a local orbital-antisymmetric spin-triplet state that can be stabilized by on-site Hund's coupling. We find that this exotic $E_g$ state can be energetically stable once a complete, realistic three-dimensional model is considered, within which momentum-dependent spin-orbit coupling terms are key. This state naturally gives rise to Bogoliubov Fermi surfaces.

We report the observation of a universal scaling of the Nernst coefficient over a wide intermediate temperature range in heavy fermion materials, including the superconductors ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$, ${\mathrm{CeCoIn}}_5$, and ${\mathrm{Ce}}_2{\mathrm{PdIn}}_8$, the ferromagnetic Kondo lattice ${\mathrm{Ce}}_3{\mathrm{RhSi}}_3$, the nonmagnetic ${\mathrm{CeRu}}_2{\mathrm{Si}}_2$, the intermediate valent ${\mathrm{YbAl}}_3$, and the hidden order compound ${\mathrm{URu}}_2{\mathrm{Si}}_2$, that cover a broad spectrum of heavy fermion materials with different crystal, valence, and ground-state properties. The scaling formula follows exactly the magnetic susceptibility of emergent heavy quasiparticles as predicted in the two-fluid model. We give a tentative explanation of the scaling based on the skew scattering mechanism and the Boltzmann picture, and we argue that the Nernst effect is produced by the asymmetry of the quasiparticle density of states rather than that of the scattering rate. In ${\mathrm{URu}}_2{\mathrm{Si}}_2$, the giant Nernst signal in the hidden order phase is also found to follow the predicted scaling, indicating the potential involvement of hybridization physics. Our work suggests a candidate unified scenario for the Nernst effect in heavy fermion materials.

We investigate the structural, electronic, vibrational, power, and transport properties of the $\beta$ allotrope of ${\mathrm{Ga}}_2{\mathrm{O}}_3$ from first principles. We find phonon frequencies and elastic constants that reproduce the correct band ordering, in agreement with experiment. We use the Boltzmann transport equation to compute the intrinsic electron and hole drift mobility and obtain room-temperature values of 258 and 1.2 ${\mathrm{cm}}^2$/Vs, respectively, as well as 6300 and 13 ${\mathrm{cm}}^2$/Vs at 100 K. Through a spectral decomposition of the scattering contribution to the inverse mobility, we find that multiple longitudinal-optical modes of $B_u$ symmetry are responsible for the electron mobility of $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ but that many acoustic modes also contribute, making it essential to include all scattering processes in the calculations. Using the von Hippel low-energy criterion, we computed the breakdown field to be 5.8 MV/cm at room temperature, yielding a Baliga figure of merit of 1250 with respect to silicon, ideal for high-power electronics. This work presents a general framework to predictively investigate novel high-power electronic materials.

Nonlinear optical response is well studied in the context of semiconductors and has gained a renaissance in studies of topological materials in the recent decade. So far it mainly deals with nonmagnetic materials and it is believed to root in the Berry curvature of the material band structure. In this work we revisit the general formalism for the second-order optical response and focus on the consequences of the time-reversal-symmetry ($\mathcal{T}$) breaking, by a diagrammatic approach. We have identified three physical mechanisms to generate a DC photocurrent, i.e., the Berry curvature, a term closely related to the quantum metric, and the diabatic motion. All three effects can be understood intuitively from the anomalous acceleration. The first two terms are respectively the antisymmetric and symmetric parts of the quantum geometric tensor. The last term is due to the dynamical antilocalization that appears from the phase accumulation between time-reversed fermion loops. Additionally, we derive the semiclassical conductivity that includes both intra- and interband effects. We find that $\mathcal{T}$ breaking can lead to a greatly enhanced nonlinear anomalous Hall effect that is beyond the contribution by the Berry curvature dipole.

In highly conductive metals with sufficiently strong momentum-conserving scattering, the electron momentum is regarded as a long-lived quantity, whose dynamics is described by an emergent hydrodynamic theory. In this paper, we develop an electron hydrodynamic theory for noncentrosymmetric metals, where a novel class of electron fluids is realized by lowering crystal symmetries and the resulting geometrical effects. The obtained hydrodynamic equation suggests a nontrivial analogy between electron fluids in noncentrosymmetric metals and chiral fluids in vacuum, and predicts novel hydrodynamic transport phenomena, that is, asymmetric Poiseuille flow and anomalous edge current. Our theory also gives a hydrodynamic description of the counterpart of various anomalous transport phenomena such as the quantum nonlinear Hall effect. Furthermore, we give a symmetry consideration on the hydrodynamic equation and propose several experimental setups to realize such anomalous hydrodynamic transport in the existing hydrodynamic materials, including bilayer graphene and Weyl semimetals.

We investigate optical absorption spectra obtained through time-dependent density functional theory (TD-DFT) based on nonempirical hybrid functionals that are designed to correctly reproduce the dielectric function. The comparison with state-of-the-art $GW$ calculations followed by the solution of the Bethe-Salpeter equation (BSE-$GW$) shows close agreement for both the transition energies and the main features of the spectra. We confront TD-DFT with BSE-$GW$ by focusing on the model dielectric function and the local exchange-correlation kernel. The present TD-DFT approach achieves the accuracy of BSE-$GW$ at a fraction of the computational cost.

Many-body localized (MBL) systems are often described using their local integrals of motion, which, for spin systems, are commonly assumed to be a local unitary transform of the set of on-site spin-$z$ operators. We show that this assumption cannot hold for topologically ordered MBL systems. Using a suitable definition to capture such systems in any spatial dimension, we demonstrate a number of features, including that MBL topological order, if present, (i) is the same for all eigenstates, (ii) is robust in character against any perturbation preserving MBL, and (iii) implies that on topologically nontrivial manifolds a complete set of integrals of motion must include nonlocal ones in the form of local-unitary-dressed noncontractible Wilson loops. Our approach is well suited for tensor-network methods and is expected to allow these to resolve highly excited finite-size-split topological eigenspaces despite their overlap in energy. We illustrate our approach on the disordered Kitaev chain, toric code, and X-cube model.

We investigated the atomic structure of metal tips by scanning individual CO molecules adsorbed on Pt(111) using scanning tunneling microscopy (STM) and frequency-modulation atomic force microscopy (FM-AFM). When scanning very close over a CO molecule, the frontmost atoms of the tip can be individually resolved in both the FM-AFM image and in the STM image. This is in contrast to previous work where CO was adsorbed on a different substrate: Cu(111). In this previous study, individual atoms could not be observed in the raw STM image but only in FM-AFM. We discuss the mechanisms behind the higher spatial resolution in STM. On Cu(111), the occupied surface state plays a large role in STM images near the Fermi level, and as adsorbed CO repels the surface state, it appears as a wide trough in STM images. In contrast, Pt(111) lacks an occupied surface state and an adsorbed CO molecule appears as a peak. We investigate if CO bending strongly influences the STM images, concluding that the atomic resolution of the tip over Pt(111) is due to highly localized through-molecule tunneling and CO bending is insignificant for contrast formation. Modelling the current between the CO and front atoms of the tip supports our findings.

We investigate the effect of charge self-consistency (CSC) in density-functional theory plus dynamical mean-field theory calculations compared to simpler “one-shot” calculations for materials where interaction effects lead to a strong redistribution of electronic charges between different orbitals or between different sites. We focus on two systems close to a metal-insulator transition (MIT), for which the importance of CSC is currently not well understood. Specifically, we analyze the strain-related orbital polarization in the correlated metal ${\mathrm{CaVO}}_3$ and the spontaneous electronic charge disproportionation in the rare-earth nickelate ${\mathrm{LuNiO}}_3$. In both cases, we find that the CSC treatment reduces the charge redistribution compared to cheaper one-shot calculations. However, while the MIT in ${\mathrm{CaVO}}_3$ is only slightly shifted due to the reduced orbital polarization, the effect of the site polarization on the MIT in ${\mathrm{LuNiO}}_3$ is more subtle. Furthermore, we highlight the role of the double-counting correction in CSC calculations containing different inequivalent sites.

We present a theoretical study of single-particle and many-body properties of twisted bilayer ${\mathrm{WSe}}_2$. For single-particle physics, we calculate the band topological phase diagram and electron local density of states (LDOS), which are found to be correlated. By comparing our theoretical LDOS with those measured by scanning tunneling microscopy, we comment on the possible topological nature of the first moiré valence band. For many-body physics, we construct a generalized Hubbard model on a triangular lattice based on the calculated single-particle moiré bands. We show that a layer potential difference, arising, for example, from an applied electric field, can drastically change the noninteracting moiré bands, tune the spin-orbit coupling in the Hubbard model, control the charge excitation gap of the Mott insulator at half-filling, and generate an effective Dzyaloshinskii-Moriya interaction in the effective Heisenberg model for the Mott insulator. Our theoretical results agree with transport experiments on the same system in several key aspects, and establish twisted bilayer ${\mathrm{WSe}}_2$ as a highly tunable system for studying and simulating strongly correlated phenomena in the Hubbard model.

A generalized theory for transport under applied electrochemical potential and temperature gradients that includes multiband effects is presented. This theory relies on a newly defined density operator for multiband wave packets, whose dynamics containing diffusive and scattering contributions are captured in a generalized equation of motion. Given that the multiband energy structure determines the transport explicitly, the case of $N$ degenerate bands within an isotropic constant relaxation time is considered as a particular case for illustration. Electric and thermal currents as well as transport properties are obtained through linear response and comparison to transport in a single band wave packet is provided. Our theory opens avenues for studying novel multiband effects and scattering processes in transport beyond the semiclassical single band approach.

We study the effect of interlayer Coulomb interaction in an electronic double layer. Assuming that each of the layers consists of a bipartite lattice, a sufficiently strong interlayer interaction leads to an interlayer pairing of electrons with a staggered order parameter. We show that the correlated pairing state is dual to the excitonic pairing state with a uniform order parameter in an electron-hole double layer. The interlayer pairing of electrons leads to strong current-current correlations between the layers. We also analyze the interlayer conductivity and the fluctuations of the order parameter, which consists of a gapped and a gapless mode.

We study the quantum phase transition upon varying the fermionic density $\nu$ in a solvable model with random Yukawa interactions between $N$ bosons and $M$ fermions, dubbed the Yukawa-SYK model. We show that there are two distinct phases in the model: an incompressible state with gapped excitations and an exotic quantum-critical, non-Fermi liquid state with exponents varying with $\nu$. We show analytically and numerically that the quantum phase transition between these two states is first-order, as for some range of $\nu$ the NFL state has a negative compressibility. In the limit $N/M\to \infty$, the first-order transition gets weaker and asymptotically becomes second-order, with an exotic quantum-critical behavior. We show that fermions and bosons display highly unconventional spectral behavior in the transition region.