We report complex band structure (CBS) calculations for the four late transition metal monoxides MnO, FeO, CoO, and NiO in their paramagnetic phase. The CBS is obtained from density functional theory plus dynamical mean field theory (DMFT) calculations to take into account correlation effects. The so-called $\beta$ parameters, governing the exponential decay of the transmission probability in the nonresonant tunneling regime of these oxides, are extracted from the CBS. Different model constructions are examined in the DMFT part of the calculation. The calculated $\beta$ parameters provide theoretical estimation for the decay length in the evanescent channel, which would be useful for tunnel junction applications of these materials.

We study the one-electron spectral properties of one-dimensional interacting electron systems in which the interactions have finite range. We employ a mobile quantum impurity scheme that describes the interactions of the fractionalized excitations at energies above the standard Tomonga-Luttinger liquid limit and show that the phase shifts induced by the impurity describe universal properties of the one-particle spectral function. We find the explicit forms in terms of these phase shifts for the momentum dependent exponents that control the behavior of the spectral function near and at the $(k,\omega )$-plane singularities where most of the spectral weight is located. The universality arises because the line shape near the singularities is independent of the short-distance part of the interaction potentials. For the class of potentials considered here, the charge fractionalized particles have screened Coulomb interactions that decay with a power-law exponent $l>5$. We apply the theory to the angle-resolved photo-electron spectroscopy (ARPES) in the highly one-dimensional bismuth-induced anisotropic structure on indium antimonide Bi/InSb(001). Our theoretical predictions agree quantitatively with both (i) the experimental value found in Bi/InSb(001) for the exponent $\alpha$ that controls the suppression of the density of states at very small excitation energy $\omega$ and (ii) the location in the $(k,\omega )$ plane of the experimentally observed high-energy peaks in the ARPES momentum and energy distributions. We conclude with a discussion of experimental properties beyond the range of our present theoretical framework and further open questions regarding the one-electron spectral properties of Bi/InSb(001).

We present an analytical low-energy theory of piezoelectric electron-phonon interactions in undoped Weyl semimetals, taking into account also Coulomb interactions. We show that piezoelectric interactions generate a long-range attractive potential between Weyl fermions. This potential comes with a characteristic angular anisotropy. From the one-loop renormalization group approach and a mean-field analysis, we predict that superconducting phases with either conventional $s$-wave singlet pairing or nodal-line triplet pairing could be realized for sufficiently strong piezoelectric coupling. For small couplings, we show that the quasiparticle decay rate exhibits a linear temperature dependence where the prefactor vanishes only in a logarithmic manner as the quasiparticle energy approaches the Weyl point. For practical estimates, we consider the Weyl semimetal TaAs.

We investigate the effect of the hexagonal warped anisotropic Fermi surface on the anomalous thermoelectric properties of the topological insulator surface illuminated with linearly polarized off-resonant electromagnetic radiation. Upon irradiation of the TI surface, the linearly polarized (LP) off-resonant radiation couples with the Dirac surface states. By explicitly finding the off-resonant quasistatic Hamiltonian we show that this coupling occurs only in the presence of the hexagonal warping of the Fermi surface of the topological insulator. We calculate the spin texture of the surface states, Berry curvature, Berry phase, orbital magnetic moment, and magnetization associated with the band structure, which we find strongly depends on the angle of polarization and the amplitude of radiation. For the anomalous thermoelectric effects generated by the Berry curvature, it is found that due to the hexagonal warping induced Fermi surface deformation and the radiation parameter dependent spin texture of the surface states, the transverse Hall, Ettingshausen, Nernst, and thermal conductivities show significant variation with the change in polarization angle, chemical potential, temperature, and the amplitude of radiation. As a consequence, the thermoelectric coefficient of performance ($ZT$) of the topological insulator is found to be increased in the presence of LP off-resonant irradiation compared to the absence of radiation.

Based on first-principles calculations, we predict a two-dimensional magnetic material, which exhibits exotic Fermi surface topology. We show that monolayer hexagonal indium carbide ($h$-InC) is thermodynamically and dynamically stable, and it energetically favors the ferromagnetic ordering of spins. The perfectly planar geometry in two dimensions, together with ferromagnetism, gives rise to a unique opportunity to encounter intriguing electronic properties, captured in the Fermi surface and band topology. We show that multiple nodal lines coexist in momentum space, accompanied by the electron and hole pockets that touch each other linearly at the nodal lines. Inclusion of spin-orbit coupling enriches the magnetic and electronic properties of $h$-InC. Spin-orbit coupling leads to an easy-plane type magnetocrystalline anisotropy, and the nodal lines can be tuned into topological nodal points, contingent upon the magnetization direction. A symmetry analysis and a tight-binding model are provided to explain the nodal structure of the bands. Our findings suggest $h$-InC as a venue for supporting carbon-based magnetism and exotic band topology in two dimensions.

We study the application of a counterdiabatic driving (CD) technique to enhance the thermodynamic efficiency and power of a quantum Otto refrigerator based on a superconducting qubit coupled to two resonant circuits. Although the CD technique is originally designed to counteract nonadiabatic coherent excitations in isolated systems, we find that it also works effectively in the open system dynamics, improving the coherence-induced losses of efficiency and power. We compare the CD dynamics with its classical counterpart, and find a deviation that arises because the CD is designed to follow the energy eigenbasis of the original Hamiltonian, but the heat baths thermalize the system in a different basis. We also discuss possible experimental realizations of our model.

Million-atom quantum simulations are in principle feasible with orbital-free density functional theory (OF-DFT) because the algorithms only require simple functional minimizations with respect to the electron density function. In this context, OF-DFT has been useful for simulations of warm dense matter, plasma, cold metals, and alloys. Unfortunately, systems as important as quantum dots and clusters (having highly inhomogeneous electron densities) still fall outside OF-DFT's range of applicability. In this Rapid Communication, we make considerable progress in addressing this century old problem by devising and implementing an accurate, transferable, and universal family of nonlocal noninteracting kinetic energy density functionals that feature correct asymptotics and can handle highly inhomogeneous electron densities. We show that OF-DFT achieves close to chemical accuracy for the electronic energy and reproduces the electron density to about 5% of the benchmark for semiconductor quantum dots and metal clusters. Therefore, this work shows that OF-DFT can reliably simulate systems with highly inhomogeneous electron density, such as clusters and quantum dots, with applicability to the rational design of materials.

We study theoretically intervalley coupling in transition metal dichalcogenide monolayers due to electron interaction with short-wavelength phonons. We demonstrate that this intervalley polaron coupling results in (i) a renormalization of the conduction band spin splitting and (ii) an increase of the electron effective masses. We also calculate the renormalization of the cyclotron energy and the Landau level splitting in the presence of an external magnetic field. An intervalley magnetophonon resonance is uncovered. Similar, but much weaker effects are also expected for the valence band holes. These results might help to resolve the discrepancy between ab initio values of the electron effective masses and the ones deduced from magnetotransport measurements.

We experimentally demonstrate an ultrastrong coupling regime between a gap plasmon and phonon in metallic-insulator-metallic antennas at far-infrared frequencies. These plasmonic antennas made of a silicon-doped semiconductor (InAsSb) are versatile structures able to reveal the polaritonic modes due to the hybridization of a gap plasmon and phonon. The anticrossing behavior featuring the ultrastrong coupling is properly studied by varying the width of the metallic antenna array. The gap-plasmon resonance supported by the antennas is shown to be particularly sensitive to the presence of phonons in the GaSb insulator. The experimental data demonstrate a giant Rabi splitting of 30% of the GaSb transverse optical phonon energy and are in good agreement with both electromagnetic and semiclassical calculations.

We survey the electrical transport properties of single-crystalline, topological chiral semimetal CoSi grown via different methods. High-quality CoSi single crystals were found in the growth from a tellurium solution. The sample's high carrier mobility enables us to observe quantum oscillations (QOs) in its thermoelectrical signals. Our analysis of QOs reveals two spherical Fermi surfaces around the $R$ point in its Brillouin zone corner. The extracted Berry phases of these electron orbits are consistent with the $-2$ chiral charge as reported in density functional theory (DFT) calculations. A detailed analysis of the QOs reveals that the spin-orbit-coupling-induced band splitting is less than 2 meV near the Fermi level, one order of magnitude smaller than our DFT calculation. We also report a large phonon-drag-induced Nernst effect in CoSi at intermediate temperatures.

A discrete nonlinear $\sigma$-model is obtained by triangulate both the space-time $M^{d+1}$ and the target space $K$. If the path integral is given by the sum of all the simplicial homomorphisms $\varphi :M^{d+1}\to K$ (i.e., maps without any topological defects), with an partition function that is independent of space-time triangulation, then the corresponding nonlinear $\sigma$-model will be called topological nonlinear $\sigma$-model which is exactly soluble. These exactly soluble models suggest that phase transitions induced by fluctuations with no topological defects usually produce a topologically ordered state and are topological phase transitions. In contrast, phase transitions induced by fluctuations with all topological defects give rise to trivial product states and are not topological phase transitions. Under the classification conjecture of Lan-Kong-Wen [Phys. Rev. X 8, 021074 (2018)], it is shown that, if $K$ is a space with only nontrivial first homotopy group $G$, which is finite, then these topological nonlinear $\sigma$-models can already realize all $3+1\text{D}$ bosonic topological orders without emergent fermions, which are described by Dijkgraaf-Witten theory with gauge group ${\pi }_1\left(K\right)=G$. Under the similar conjecture, we show that the $3+1\text{D}$ bosonic topological orders with emergent fermions can be realized by topological nonlinear $\sigma$-models with ${\pi }_1\left(K\right)=$ finite groups, ${\pi }_2\left(K\right)=Z_2$, and ${\pi }_{n>2}\left(K\right)=0$. A subset of these topological nonlinear $\sigma$-models corresponds to 2-gauge theories, which realize and may classify bosonic topological orders with emergent fermions that have no emergent Majorana zero modes at triple string intersections.

We have investigated many-body renormalizations of the single-particle excitations in $1T\text{−}{\mathrm{TiSe}}_2$ by employing high resolution angle-resolved photoemission spectroscopy (ARPES) measurements. The energy distribution curves (EDCs) of the ARPES data reveal an intrinsic single band peak-dip-hump (PDH) feature. Furthermore, the renormalized electronic dispersion extracted from the momentum distribution curves (MDCs) highlights a well-defined kink structure. These are canonical signatures of many-body correlations in the system. Theoretical modeling of the electrons coupled to an Einstein mode illustrates that a study of the renormalized dispersion from the MDCs enable direct access to the characteristic features of these many-body correlations, such as the energy scale of the relevant collective mode and the strength of its coupling with the electrons in the system. This model also demonstrates the difficulty to determine these features in a straightforward way from the PDH structure of the EDCs. The self-energy analysis of our ARPES data suggest compelling evidence for a bosonic mode having energy $\sim 26\mathrm{meV}$, with which the electrons in $1T\text{−}{\mathrm{TiSe}}_2$ couple to. This correlates with the ab initio phonon-dispersion calculations and the observation of breathing ($A_{1g}$) phonon mode in Raman scattering experiments.

We study the quantum topological properties of Floquet (time-periodic) systems exhibiting Hall effects due to perpendicular magnetic and electric fields. The systems are charged particles periodically kicked by a one-dimensional cosine potential in the presence of such fields, where the magnetic field is perpendicular also to the kicking direction. We consider parameter values including small kicking strength, which enables the quantum evolution to be described by effective Floquet Hamiltonians. We also assume the semiclassical regime of a small scaled Planck constant. In the case of an electric field parallel to the kicking direction, one observes, as the electric-field strength is varied, a series of topological phase transitions, causing changes in the Chern numbers of quasienergy (QE) bands. These transitions are due to band degeneracies which, in an initial wide range of electric-field strengths, are shown to occur when the energy of the classical separatrix is approximately equal to the average QE of the degenerating bands. For larger electric-field strengths, the degeneracies reflect changes in the classical phase-space structure. In the case of an electric field not parallel to the kicking direction and satisfying some resonance conditions, we show that the topological properties of the QE bands are characterized by universal (electric-field independent) Chern numbers. These same numbers also characterize the QE band spectrum in the first case, at the end of the basic electric-field interval where the topological phase transitions take place. The two cases above are known to exhibit significantly different dynamical rates, both classical and quantum. This is well reflected by the different topological properties in the two cases.

We report electrical transport measurements on ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$ single crystals under applied pressure. At ambient pressure and high temperatures, ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$ behaves as a single-band semimetal hosting Néel order at $T_N=150$ K. At low temperatures, multiband behavior emerges along with an activated behavior typical of degenerate semiconductors. The activation gap is estimated to be $\mathrm{\Delta }\sim 20$ K. Applied pressure not only favors the antiferromagnetic order at a rate of 0.40(2) K/kbar, but it also enhances the activation gap at 20 kbar by about 70%. This gap enhancement is typical of correlated narrow-gap semiconductors such as FeSi and ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pt}}_3$, and it places ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$ as a Mn-based hybridization-gap semiconductor candidate. Ab initio calculations based on density functional theory are shown to be insufficient to describe the ground state of ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$.

Noncentrosymmetric metals such as ${\mathrm{Li}}_2{\left({\mathrm{Pd}}_{1-x}{\mathrm{Pt}}_x\right)}_3\mathrm{B}$ have different Fermi surface topology below and above the band touching point where spin-degeneracy is not lifted by the spin-orbit coupling. We investigate thermoelectric and optical response as probes for this Fermi surface topology change. We show that the chemical potential displays a dimensional crossover from three-dimensional to one-dimensional characteristics as the descending Fermi energy crosses the band touching point. This dimensional crossover is due to the existence of different Fermi surface topology above and below the band touching point. We obtain an exact expression of relaxation time due to short-range scatterer by solving Boltzmann transport equations self-consistently. The thermoelctric power and figure of merit are significantly enhanced as the Fermi energy goes below the band touching point owing to the underlying one-dimensional-like nature of noncentrosymmteric bulk metals. The value of thermoelectric figure of merit goes beyond two as the Fermi energy approaches to the van Hove singularity for lower spin-orbit coupling. Similarly, the studies of the zero-frequency and finite-frequency optical conductivities in the zero-momentum limit reflect the nature of topological change of the Fermi surface. The Hall coefficient and optical absorption width exhibit distinct signatures in response to the changes in Fermi surface topology.

We study the spin conductivity of the surface states in a thin film of a topological insulator within Kubo formalism. Hybridization between the different sides of the film opens a gap at the Dirac point. We found that in the gapped region spin conductivity remains finite. In the gapless region near the band gap, spin conductivity is enhanced. These findings indicate that a thin film of a topological insulator is a promising material for spintronic applications.

We control the thickness of GaSe on the level of individual layers and study the corresponding optical absorption via highly sensitive differential transmission measurements. Suppression of excitonic transitions is observed when the number of layers is smaller than a critical value of 8. Through ab initio modelling we are able to link this behavior to a fundamental change in the band structure that leads to the formation of a valence band shaped as an inverted Mexican hat in thin GaSe. The thickness-controlled modulation of the optical properties provides attractive resources for the development of functional optoelectronic devices based on a single material.

Ferroelectric vortices are promising candidates for developing high-capacity memory devices and discovery of intriguing electromechanical properties. It is well known that the ferroelectric vortex state tends to stabilize at poor screening conditions and a relatively good screening condition would give rise to a polar state. However, what happens at the intermediate screening condition remains unclear. In this work, by including a Bardeen screening model into phase-field simulations, we explore the screening-mediated state stabilization in ferroelectric nanodots. A state coexistence regime where the polar and toroidal orders can both stabilize in a ferroelectric nanodot is found at certain range of screening (that is, screening length from 0.011 to 0.022 nm at room temperature). The mechanism of state coexistence is analyzed by construction of the input-dependent free-energy profile according to the energy minimum principle. The coexistence regime is further revealed to be tunable by geometrical parameters. Importantly, the polar and vortex states can be readily switched between each other via electrical and mechanical loads, paving the way towards a type of nonvolatile multilevel memory. Moreover, a screening-mediated morphotropic phase boundary (MPB) with strong dielectric and piezoelectric responses is demonstrated. Our study thus discovers a stability crossover of polar and toroidal orders in ferroelectric nanodots at intermediate screening condition accompanied with superior electromechanical properties and the feasibility in multilevel memory application.

We systematically investigate the influence of high pressure on the electronic transport properties of layered ferromagnetic materials; in particular, those of ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$. Its crystal sustains a hexagonal phase under high pressures up to 25.9 GPa, while the Curie temperature decreases monotonously with the increasing pressure. By applying appropriate pressures, the experimentally measured anomalous Hall conductivity, ${\sigma }_{xy}^A$, can be efficiently controlled. Our theoretical study reveals that this finding can be attributed to the shift of the spin–orbit-coupling-induced splitting bands of Fe atoms. With loading compression, ${\sigma }_{xy}^A$ reaches its maximal value when the Fermi level lies inside the splitting bands and then attenuates when the splitting bands float above the Fermi level. Further compression leads to a prominent suppression of the magnetic moment, which is another physical cause of the decrease in ${\sigma }_{xy}^A$ at high pressure. These results indicate that the application of pressure is an effective approach in controlling the anomalous Hall conductivity of layered magnetic materials, which elucidates the physical mechanism of the large intrinsic anomalous Hall effect.

We theoretically show that a temporally oscillating spin-driven electromotive force or voltage with a large DC component can be generated by exciting microwave-active spin-wave modes of a skyrmion crystal confined in a quasi-two-dimensional magnet under a tilted magnetic field. The DC component and the AC amplitude of the oscillating electric voltage is significantly enhanced when the microwave frequency is tuned to an eigenfrequency of the peculiar spin-wave modes of the skyrmion crystal called “rotation modes” and the “breathing mode,” whereas the sign of the DC component depends on the microwave polarization and on the spin-wave mode. These results provide an efficient means to convert microwaves to a DC electric voltage by using skyrmion-hosting magnets and to switch the sign of the voltage via tuning the microwave frequency, which are important capabilities for spintronic devices.