Weyl semimetal with Weyl fermions at the Fermi energy is one of the topological materials and is a condensed-matter realization of the relativistic fermions. However, there are several crucial differences such as the shift of the Fermi energy, which can hinder the expected interesting physics. Chiral anomaly is a representative nontrivial phenomenon associated with Weyl fermions, which dictates the transfer of fermions between the Weyl fermions with opposite chirality; it is manifested as the negative magnetoresistance. Here we demonstrate that the magnetoresistance is robust against the deviation from the ideal Weyl Hamiltonian such as the shifted Fermi energy and nonlinear dispersions. We study a model with the energy dispersion containing two Weyl nodes, and find that the magnetoresistance persists even when the Fermi level is far away from the node, even above the saddle point that separates the two nodes. Surprisingly, the magnetoresistance remains even after the pair annihilation of the nodes.

van der Waals (vdW) bilayers have many attractive novel properties that offer an ideal platform for various electronic and optoelectronic applications. However, due to the strong interlayer overlapping, the electron-hole recombination lifetime in vdW bilayers is usually very short, which limits their applications. Here, based on the time-dependent density functional theory combined with nonadiabatic molecular dynamics, we demonstrate that the Janus-MoSSe bilayer can have a staggered band gap, in which the maximum valence and minimum conduction bands are spatially separated on different layers. Such type-II band alignment is surprisingly robust against external perturbations, such as twisting or stacking modes, which can be ascribed to the intrinsic out-of-plane electric field. Further analysis indicates that due to the existence of such a built-in electric field, the overlapping within the interlayer is suppressed, leading to the completely spacial detachment of the electrons and holes. We then evaluate the photogenerated electron and hole recombination dynamics. The predicted recombination time is extremely long, with a value up to 16.5 ns, which is even longer than that of the ${\mathrm{MoS}}_2/{\mathrm{WS}}_2$ vdW heterostructure. Our findings propose that introducing an intrinsic electric field is an efficient way to tune the electronic structures of bilayers, which can significantly extend the recombination time of a photoexcited electron and hole, and thus facilitate the design of advanced two-dimensional semiconducting devices.

We apply scanning tunneling spectroscopy to determine the band gaps of mono-, bi-, and trilayer ${\mathrm{MoS}}_2$ grown on a graphene single crystal on Ir(111). Besides the typical scanning tunneling spectroscopy at constant height, we employ two additional spectroscopic methods giving extra sensitivity and qualitative insight into the $k$ vector of the tunneling electrons. Employing this comprehensive set of spectroscopic methods in tandem, we deduce a band gap of $2.53\pm 0.08$ eV for the monolayer. This is close to the predicted values for freestanding ${\mathrm{MoS}}_2$ and larger than is measured for ${\mathrm{MoS}}_2$ on other substrates. Through precise analysis of the “comprehensive” tunneling spectroscopy we also identify critical point energies in the mono- and bilayer ${\mathrm{MoS}}_2$ band structures. These compare well with their calculated freestanding equivalents, evidencing the graphene/Ir(111) substrate as an excellent environment upon which to study the many celebrated electronic phenomena of monolayer ${\mathrm{MoS}}_2$ and similar materials. Additionally, this investigation serves to expand the fledgling field of the comprehensive tunneling spectroscopy technique itself.

We investigate two-dimensional hole transport in GaSb quantum wells at cryogenic temperatures using gate-tunable devices. Measurements probing the valence band structure of GaSb unveil a significant spin splitting of the ground subband induced by spin-orbit coupling. We characterize the carrier densities, effective masses, and quantum scattering times of these spin-split subbands and find that the results are in agreement with band structure calculations. Additionally, we study the weak antilocalization correction to the conductivity present around zero magnetic field and obtain information on the phase coherence. These results establish GaSb quantum wells as a platform for two-dimensional hole physics and lay the foundations for future experiments in this system.

We develop a rigorous theory for nanophotonic cooperative spontaneous emission of an ensemble of quantum emitters near matter, which also incorporates absorption. Such theory is highly requested since one should expect that in the field of nanophotonics the close-by emitters will emit cooperatively simultaneously with strong interaction with the exotic metamaterial complex dielectric function. The closed-form model developed here considers the buildup both of interemitter correlation over time through emitter transitions via the mutual field and of geometry-induced spatial correlation. The presence of matter alters the local density of states (LDOS), resulting in modified decay rate and emission intensity for the entire ensemble relative to values of spontaneous emission in free space. The superradiant total emission is separable into a quantum-mechanical time-dependent part and a classical space-dependent part, both being functions of the LDOS. Two distinct radiative emitter–far-field coupling mechanisms, which together account for the superradiant emission, are obtained—the coupling of every individual emitter with the far field and the coupling of emitter pairs to the far field. The calculations for superradiating emitters near silver and near-zero epsilon (NZE) materials were performed with the following results: the silver sphere augments the superradiant far-field intensity and rate by up to 400 and 1000%, respectively, while the NZE sphere enhances the far-field intensity and rate of the superradiance by approximately 1400 and 400%, respectively.

A wide range of disordered materials, including disordered correlated systems, show universal dielectric response (UDR), followed by a superlinear power-law increase in their optical responses over exceptionally broad frequency regimes. While extensively used in various contexts over the years, the microscopics underpinning UDR remains controversial. Here, we investigate the optical response of the simplest model of correlated fermions, the Falicov-Kimball model, across the continuous metal-insulator transition (MIT) and analyze the associated quantum criticality in detail using cluster extension of dynamical mean-field theory. Surprisingly, we find that UDR naturally emerges in the quantum critical region associated with the continuous MIT. We tie the emergence of these novel features to a many-body orthogonality catastrophe accompanying the onset of strongly correlated electronic glassy dynamics close to the MIT, providing a microscopic realization of Jonscher's time-honored proposal as well as a rationale for similarities in optical responses between correlated electronic matter and canonical glass formers.

We report a plasmon-polaron mode of a two-dimensional electron gas occupying surface states of a three-dimensional topological crystal. This low-frequency, acoustic plasmon mode splits off from the conventional spin-plasmon mode as a result of strong interactions of the surface electrons with bulk phonons. We show that like in the case of the conventional spin plasmon, the scattering of this mode is strongly suppressed in some regions of the phase space. This signature of topological protection leads to an umklapp-free mode dispersion at the Brillouin zone boundary. Such a plasmon-polaron mode has indeed been recently observed in the topological metal ${\mathrm{Bi}}_2{\mathrm{Se}}_3$.

${\mathrm{CaCu}}_3{\mathrm{Ti}}_4{\mathrm{O}}_{12}$ (CCTO) has a large dielectric permittivity plateau near room temperature due to several dynamic processes. Here, we consider the combined effects of localized charge carriers (polarons) and conductive charge carriers using a recently proposed statistical model [Phys. Rev. B 96, 054115 (2017)] to fit and understand its permittivity measured at different frequencies over a broad temperature range. We found that, at the lowest temperature, the small permittivity is related to frozen polarons, and the increase at higher temperatures is associated with the thermal excitation of polarons that gives rise to the Maxwell-Wagner effect. The final rapid increase at the highest temperature is attributed to thermally activated conductivity. Such an analysis enables us to separate the contributions from localized polarons and conductive charge carriers and quantify their activation energies, which also explains the permittivity plateau near room temperature. In particular, we show that the subtle balance between the number of activated polarons and their polarizability causes CCTO to have a permittivity plateau with small dielectric loss.

Geometrically frustrated systems and topological semimetals have both attracted much interest and been studied in various systems in recent years. Here we study the interplay between these two systems. We show that a Weyl point can be extended to a chain of degeneracy (a nodal chain) with nonzero charge of Berry flux through geometric frustration. We propose to realize such a charged nodal chain in an acoustic metamaterial, based on both tight-binding and full-wave numerical simulations. Moreover, we observe a fanlike surface state spectrum, the dispersion of which is controlled by the bulk band properties. Our paper points to a class of band degeneracy that carries nonzero Berry flux. The resulting topological metamaterial may be useful for controlling the flow of sound and light.

Study of friction and energy dissipation always relied on direct observations. Actual theories provide limited prediction on the frictional and dissipative properties if only the material chemistry and geometry are known. We here develop a framework to study intrinsic friction and energy dissipation based on the only knowledge of the normal modes of the system at equilibrium. We derive an approximated expression for the first anharmonic term in the potential energy expansion which does not require the computation of the third-order force constants. Moreover, we show how to characterize the frequency content of observed physical quantities and individuate the dissipative processes active during experimental measurements. As a case study, we consider the relative sliding motion of atomic layers in molybdenum disulfide dry lubricant, and we discuss how to extract information on the energetics of sliding from atomic force microscopy signals. The presented framework switches the investigation paradigm on friction and energy dissipation from dynamic to static studies, paving avenues to explore for the design of alternative anisotropic tribological and thermal materials.

We have simulated the ultrahigh-temperature ceramic zirconium carbide (ZrC) in order to predict electron and phonon scattering properties, including lifetimes and transport. Our predictions of heat and charge conductivity, which extend to 3000 K, are relevant to extreme-temperature applications of ZrC. Mechanisms are identified on a first-principles basis that considerably enhance or suppress heat transport at high temperature, including strain, anharmonic phonon renormalization, and four-phonon scattering. The extent to which boundary confinement and isotope scattering effects lower thermal conductivity is predicted.

Recent work [Martin et al., Phys. Rev. X 7, 041008 (2017)] shows that a spin coupled to two externally supplied circularly polarized electromagnetic modes can effectuate a topological, quantized transfer of photons from one mode to the other. Here, we study the effect in the case when only one of the modes is externally provided, while the other is a dynamical quantum mechanical cavity mode. Focusing on the signatures and stability under experimentally accessible conditions, we show that the effect persists down to the few-photon quantum limit and that it can be used to generate highly entangled “cat states” of cavity and spin. By tuning the strength of the external drive to a “sweet spot,” the quantized pumping can arise starting from an empty (zero-photon) cavity state. We also find that inclusion of external noise and dissipation does not suppress but rather stabilizes the conversion effect, even after multiple cavity modes are taken into account.

We report on a detailed neutron diffraction and ${}^1\mathrm{H}$-NMR study on the frustrated spin-1/2 chain material linarite, ${\mathrm{PbCuSO}}_4{\left(\mathrm{OH}\right)}_2$, where competing ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions lead to frustration. From the magnetic Bragg peak intensity studied down to 60 mK, the magnetic moment per Cu atom is obtained within the whole magnetic phase diagram for $H\parallel b$ axis. Further, we establish the detailed configurations of the shift of the SDW propagation vector in phase V with field and temperature. Finally, combining our neutron diffraction results with those from a low-temperature/high-field NMR study, we find an even more complex phase diagram close to the quasisaturation field suggesting that bound two-magnon excitations are the lowest energy excitations close to and in the quasisaturation regime. Qualitatively and semiquantitatively, we relate such behavior to $XYZ$ exchange anisotropy and contributions from the Dzyaloshinsky-Moriya interaction to affect the magnetic properties of linarite.

The high-pressure phases and superconductivity of ${\mathrm{CaYH}}_{12}$ have been explored by using a particle swarm optimization structure prediction methodology in combination with first-principles calculations. Our results show that ${\mathrm{CaYH}}_{12}$ becomes stable with a cubic $Fd\overline{3}m$ structure above 170 GPa, where metal atoms form body-centered-cubic (bcc) lattices and hydrogens occupy all the tetrahedral interstices of these bcc lattices, completing sodalitelike cages. The electron-phonon coupling calculations indicate that the $Fd\overline{3}m$ structure is a potential high-temperature superconductor, with a calculated $T_c$ of 258 K at 200 GPa. Our current study provides a possibility for searching new high-$T_c$ superconductors in ternary hydrides.

The anomalous Hall effect (AHE) and the magneto-optical effect (MOE) are two prominent manifestations of time-reversal symmetry breaking in magnetic materials. Noncollinear antiferromagnets (AFMs) have recently attracted a lot of attention owing to the potential emergence of exotic spin orders on geometrically frustrated lattices, which can be characterized by corresponding spin chiralities. By performing first-principles density functional calculations together with group-theory analysis and tight-binding modeling, here we systematically study the spin-order dependent AHE and MOE in representative noncollinear AFMs ${\mathrm{Mn}}_{3}X\mathrm{N}(X=\mathrm{Ga}$, Zn, Ag, and Ni). The symmetry-related tensor shape of the intrinsic anomalous Hall conductivity (IAHC) for different spin orders is determined by analyzing the relevant magnetic point groups. We show that while only the $xy$ component of the IAHC tensor is nonzero for right-handed spin chirality, all other elements—${\sigma }_{xy},{\sigma }_{yz}$, and ${\sigma }_{zx}$—are nonvanishing for a state with left-handed spin chirality owing to lowering of the symmetry. Our tight-binding arguments reveal that the magnitude of IAHC relies on the details of the band structure and that ${\sigma }_{xy}$ is periodically modulated as the spin rotates in-plane. The IAHC obtained from first principles is found to be rather large, e.g., it amounts to 359 S/cm in ${\mathrm{Mn}}_3\mathrm{AgN}$, which is comparable to other well-known noncollinear AFMs such as ${\mathrm{Mn}}_3\mathrm{Ir}$ and ${\mathrm{Mn}}_3\mathrm{Ge}$. We evaluate also the magnetic anisotropy energy and find that the evolution of spin order is related to the number of valence electrons in the $X$ ion. Interestingly, the left-handed spin chirality could exist in ${\mathrm{Mn}}_{3}X\mathrm{N}$ with some particular spin configurations. By extending our analysis to finite frequencies, we calculate the optical isotropy $[{\sigma }_{xx}\left(\omega \right)\approx {\sigma }_{yy}\left(\omega \right)\approx {\sigma }_{zz}\left(\omega \right)]$ and the magneto-optical anisotropy $[{\sigma }_{xy}\left(\omega \right)\ne {\sigma }_{yz}\left(\omega \right)\ne {\sigma }_{zx}\left(\omega \right)]$ of ${\mathrm{Mn}}_{3}X\mathrm{N}$. Similar to the IAHC, the magneto-optical Kerr and Faraday spectra depend strongly on the spin order. The Kerr rotation angles in ${\mathrm{Mn}}_{3}X\mathrm{N}$ are in the range of $0.3^{\circ }\sim 0.4^{\circ }$, which is large and comparable to other noncollinear AFMs like ${\mathrm{Mn}}_3\mathrm{Pt}$ and ${\mathrm{Mn}}_3\mathrm{Sn}$. Our finding of large AHE and MOE in ${\mathrm{Mn}}_{3}X\mathrm{N}$ suggests that these materials present an excellent antiferromagnetic platform for realizing novel spintronics and magneto-optical devices. We argue that the spin-order dependent AHE and MOE are indispensable in detecting complex spin structures in noncollinear AFMs.

Spin gapless semiconductors (SGSs) are an interesting class of materials which bridge the gap between semiconductors and half-metallic ferromagnets. This class of materials shows band gap in one of the spin channels and a zero band gap in the other, and thus promote tunable spin transport. Here, we present structural, electronic, magnetic, and transport properties of Co-rich SGS ${\mathrm{Co}}_{1+x}{\mathrm{Fe}}_{1-x}\mathrm{CrGa}$ using both theoretical and experimental techniques. The key advantage of Co-rich samples ${\mathrm{Co}}_{1+\mathit{x}}{\mathrm{Fe}}_{1-\mathit{x}}\mathrm{CrGa}$ is the high Curie temperature $\left(T{}_C\right)$ and magnetization, without compromising the SGS nature (up to $x=0.4)$, and hence our choice. The quaternary Heusler alloys ${\mathrm{Co}}_{1+\mathit{x}}{\mathrm{Fe}}_{1-\mathit{x}}\mathrm{CrGa}(x=0.1$ to 0.5) are found to crystallize in LiMgPdSn-type structures having space group $F\overline{4}3m$ (number of 216). The measured Curie temperature increases from 690 K $\left(x=0\right)$ to 870 K $\left(x=0.5\right)$. The obtained ${\mathrm{T}}_{\mathrm{C}}$ for $x=0.3$ (790 K) is found to be the highest among all previously reported SGS materials. Observed magnetization values follow the Slater-Pauling rule. Measured electrical resistivity, in the temperature range of 5–350 K, suggests that the alloys retain the SGS behavior up to $x=0.4$, beyond which it reflects metallic character. Unlike conventional semiconductors, the conductivity value $\left({\sigma }_{\mathit{xx}}\right)$ at 300 K lies in the range of 2289 S ${\mathrm{cm}}^{-1}$ to 3294 S ${\mathrm{cm}}^{-1}$, which is close to that of other reported SGS materials. The anomalous Hall effect is comparatively low. The intrinsic contribution to the anomalous Hall conductivity increases with $x$, which can be correlated with the enhancement in chemical order. The anomalous Hall coefficient is found to increase from 38 S/cm for $x=0.1$ to 43 S/cm for 0.3. Seebeck coefficients turn out to be vanishingly small below 300 K, another signature for being SGS. All the alloys (for different $x)$ are found to be both chemically and thermally stable. Simulated magnetization agrees fairly with the experiment. As such, Co-rich CoFeCrGa is a promising candidate for room temperature spintronic applications, with enhanced $T{}_C$, magnetic properties, and SGS nature.

We present the concept, derivation, and implementation of dynamical configuration interaction, a quantum embedding theory that combines Green's function methodology with the many-body wave function. In a strongly correlated active space, we use full configuration interaction (CI) to describe static correlation exactly. We add energy-dependent corrections to the CI Hamiltonian which, in principle, include all remaining correlations derived from the bath space surrounding the active space. Next, we replace the exact Hamiltonian in the bath with one of excitations defined over a correlated ground state. This transformation is naturally suited to the methodology of many-body Green's functions. In this space, we use a modified $GW$/Bethe-Salpeter equation procedure to calculate excitation energies. Combined with an estimate of the ground-state energy in the bath, we can efficiently compute the energy-dependent corrections, which correlate the full set of orbitals, for very low computational cost. We present dimer dissociation curves for ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$ in good agreement with exact results. Additionally, excited states of ${\mathrm{N}}_2$ and ${\mathrm{C}}_2$ are in excellent agreement with benchmark theory and experiment. By combining the strengths of two disciplines, we achieve a balanced description of static and dynamic correlation in a fully ab initio, systematically improvable framework.