We utilize variational method to investigate the Kondo screening of a spin-1/2 magnetic impurity in tilted Dirac surface states with the Dirac cone tilted along the $k_y$ axis. We mainly study about the effect of the tilting term on the binding energy and the spin-spin correlation between magnetic impurity and conduction electrons. The binding energy has a critical value while the Dirac cone is slightly tilted. However, as the tilting term increases, the density of states near the Dirac node becomes significant, such that the impurity and the host material always favor a bound state. The diagonal and the off-diagonal terms of the spin-spin correlation between the magnetic impurity and conduction electrons are also studied. Due to the spin-orbit coupling and the tilting of the spectra, various components of spin-spin correlation show very strong anisotropy in coordinate space and are of power-law decay with respect to the spatial displacements.

Nuclear spin-lattice ($1/T_1$) and spin-spin ($1/T_2$) relaxation rates of the cation sites of a candidate quantum spin-liquid ${\beta }^{\prime }-{\mathrm{EtMe}}_3\mathrm{Sb}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ and its deuterated sample are presented. The enhanced $1/T_1$ of ${}^1\mathrm{H}$ and ${}^2\mathrm{D}$ is thoroughly analyzed considering the rotations of methyl and ethyl groups of the cation with activation energies of $2\times {10}^2$ K and $1.2\times {10}^3$ K respectively. Contrasting charge dynamics with an algebraic temperature dependence is found at the Sb site in the ${\mathrm{EtMe}}_3\mathrm{Sb}$ cation. The charge fluctuation remains active down to the lowest temperature, as has been observed in the ac response of dielectric constants.

The nonmagnetic compounds showing extremely large magnetoresistance are attracting a great deal of research interest due to their potential applications in the field of spintronics. ${\mathrm{PtBi}}_2$ is one of such interesting compounds showing large linear magnetoresistance (MR) in both the hexagonal and pyrite crystal structure. We use angle-resolved photoelectron spectroscopy and density functional theory calculations to understand the mechanism of liner MR observed in the layered ${\mathrm{PtBi}}_2$. Our results uncover linear dispersive surface Dirac states at the $\overline{\mathrm{\Gamma }}$ point, crossing the Fermi level with a node at a binding energy of $\approx 900$ meV, in addition to the previously reported Dirac states at the $\overline{M}$ point in the same compound. We further notice from our dichroic measurements that these surface states show an asymmetric spectral intensity when measured with left and right circularly polarized light, hinting at a substantial spin polarization of the bands. Following these observations, we suggest that the linear dispersive Dirac states at the $\overline{\mathrm{\Gamma }}$ and $\overline{M}$ points are likely to play a crucial role for the linear field dependent magnetoresistance recorded in this compound.

We report a comprehensive angle-resolved photoemission spectroscopy study of NaFeAs, a prototypical parent compound of the Fe-based superconductors. By mechanically detwinning the samples, we show that in the nematic phase (below the structural transition at $T_s=54$ K but above the antiferromagnetic transition at $T_N=43$ K) spectral weight is detected on only the elliptical electron pocket along the longer $a_{\text{orth}}$ axis. This dramatic anisotropy is likely to arise as a result of coupling to a fluctuating antiferromagnetic order in the nematic phase. In the long-range ordered antiferromagnetic state below $T_N$, this single electron pocket is backfolded and hybridizes with the hole bands, leading to the reconstructed Fermi surface. By careful analysis of the $k_z$ variation, we show that the backfolding of spectral weight in the magnetic phase has a wave vector of ($\pi ,0,\pi$), with the $c$-axis component being in agreement with the magnetic ordering in NaFeAs observed by neutron scattering. Our results clarify the origin of the tiny Fermi surfaces of NaFeAs at low temperatures and highlight the importance of the three-dimensional aspects of the electronic and magnetic properties of Fe-based superconductors.

Ultrafast perturbations offer a unique tool to manipulate correlated systems due to their ability to promote transient behaviors with no equilibrium counterpart. A widely employed strategy is the excitation of coherent optical phonons, as they can cause significant changes in the electronic structure and interactions on short time scales. One of the issues, however, is the inevitable heating that accompanies these resonant excitations. Here, we explore a promising alternative route: the nonequilibrium excitation of acoustic phonons, which, due to their low excitation energies, generally lead to less heating. We demonstrate that driving acoustic phonons leads to the remarkable phenomenon of a momentum-dependent effective temperature, by which electronic states at different regions of the Fermi surface are subject to distinct local temperatures. Such an anisotropic effective electronic temperature can have a profound effect on the delicate balance between competing ordered states in unconventional superconductors, opening a so far unexplored avenue to control correlated phases.

The magnetic and transport properties of a conducting layer with Rashba spin-orbit coupling (RSOC) magnetically coupled to a layer of localized magnetic moments are studied on strips of varying width. The localized moments are free to rotate and they acquire an order that results from the competition between the magnetic exchange energy and the kinetic energy of the conduction electrons. By minimizing the total Hamiltonian within the manifold of variational spiral orders of the magnetic moments, the phase diagram in the space of the interlayer exchange $J_{\mathrm{sd}}$ and the ratio of the Rashba coupling to the hopping integral $\lambda /t$ was determined. Two main phases with longitudinal spiral order were found, one at large interlayer coupling $J_{\mathrm{sd}}$ with uniform order in the transversal direction, and the other at small $J_{\mathrm{sd}}$ showing a transversal staggered order. This staggered spiral order is unstable against an antiferromagnetic (AFM) one for large values of $\lambda /t$. In both spiral phases, the longitudinal spiral momentum departs from the expected linear dependence with the RSOC for large values of $\lambda /t$. Then, various transport properties, including the longitudinal Drude weight and the spin Hall conductivity, inside these two phases are computed in linear response, and their behavior is compared with the ones for the more well-studied cases of a fixed ferromagnetic (FM) and AFM localized magnetic orders.

We present ab initio calculations of the photoemission spectra of bulk sodium using different flavors of the cumulant expansion approximation for the Green's function. In particular, we study the dispersion and intensity of the plasmon satellites. We show that the satellite spectrum is much more sensitive to many details than the quasiparticle spectrum, which suggests that the experimental investigation of satellites could yield additional information beyond the usual studies of the band structure. In particular, a comparison to the homogeneous electron gas shows that the satellites are influenced by the crystal environment, although the crystal potential in sodium is weak. Moreover, the temperature dependence of the lattice constant is reflected in the position of the satellites. Details of the screening also play an important role; in particular, the contribution of transitions from $2s$ and $2p$ semicore levels influences the satellites, but not the quasiparticle. Moreover, inclusion of contributions to the screening beyond the random-phase approximation has an effect on the satellites. Finally, we elucidate the importance of the coupling of electrons and holes by comparing the results of the time-ordered and the retarded cumulant expansion approximations. Again, we find small but noticeable differences. Since all the small effects add up, our most advanced calculation yields a satellite position which is improved with respect to previous calculations by almost 1 eV. This stresses the fact that the calculation of satellites is much more delicate than the calculation of a quasiparticle band structure.

The conventional theory of solids is well suited to describing band structures locally near isolated points in momentum space, but struggles to capture the full, global picture necessary for understanding topological phenomena. In part of a recent paper [B. Bradlyn et al., Nature (London) 547, 298 (2017)], we have introduced the way to overcome this difficulty by formulating the problem of sewing together many disconnected local $\mathbf{k}·\mathbf{p}$ band structures across the Brillouin zone in terms of graph theory. In this paper, we give the details of our full theoretical construction. We show that crystal symmetries strongly constrain the allowed connectivities of energy bands, and we employ graph theoretic techniques such as graph connectivity to enumerate all the solutions to these constraints. The tools of graph theory allow us to identify disconnected groups of bands in these solutions, and so identify topologically distinct insulating phases.

The link between chemical orbitals described by local degrees of freedom and band theory, which is defined in momentum space, was proposed by Zak several decades ago for spinless systems with and without time reversal in his theory of “elementary” band representations. In a recent paper [Bradlyn et al., Nature (London) 547, 298 (2017)] we introduced the generalization of this theory to the experimentally relevant situation of spin-orbit coupled systems with time-reversal symmetry and proved that all bands that do not transform as band representations are topological. Here we give the full details of this construction. We prove that elementary band representations are either connected as bands in the Brillouin zone and are described by localized Wannier orbitals respecting the symmetries of the lattice (including time reversal when applicable), or, if disconnected, describe topological insulators. We then show how to generate a band representation from a particular Wyckoff position and determine which Wyckoff positions generate elementary band representations for all space groups. This theory applies to spinful and spinless systems, in all dimensions, with and without time reversal. We introduce a homotopic notion of equivalence and show that it results in a finer classification of topological phases than approaches based only on the symmetry of wave functions at special points in the Brillouin zone. Utilizing a mapping of the band connectivity into a graph theory problem, we show in companion papers which Wyckoff positions can generate disconnected elementary band representations, furnishing a natural avenue for a systematic materials search.

The Sauter-Schwinger effect predicts the creation of electron-positron pairs out of the quantum vacuum via tunneling induced by a strong electric field. Unfortunately, as the required field strength is extremely large, this fundamental prediction of quantum field theory has not been verified experimentally yet. Here, we study under which conditions and approximations the interband tunneling in suitable semiconductors could be effectively governed by the same (Dirac) Hamiltonian, especially for electric fields which depend on space and time. This quantitative analogy would allow us to test some of the predictions (such as the dynamically assisted Sauter-Schwinger effect) in this area by means of these laboratory analogs.

Electron-phonon coupling is known to play an important role in the charge dynamics of semiconductor quantum dots. Here we explore its role in the combined charge-photon dynamics of cavity-coupled double quantum dots. Previous work on these systems has shown that strong electron-phonon coupling leads to a large contribution to photoemission and gain from phonon-assisted emission and absorption processes. We compare the effects of this phonon sideband in three commonly investigated gate-defined quantum dot material systems: InAs nanowires and GaAs and Si two-dimensional electron gases (2DEGs). We compare our theory with existing experimental data from cavity-coupled InAs nanowire and GaAs 2DEG double quantum dots and find quantitative agreement only when the phonon sideband and photoemission processes during lead tunneling are taken into account. Finally, we show that the phonon sideband also leads to a sizable renormalization of the cavity frequency, which allows for direct spectroscopic probes of the electron-phonon coupling in these systems.

We develop an analytical mode-matching technique for the tight-binding model to describe electron transport across graphene P-N junctions. This method shares the simplicity of the conventional mode-matching technique for the low-energy continuum model and the accuracy of the tight-binding model over a wide range of energies. It further reveals an interesting phenomenon on a sharp P-N junction: the disappearance of the well-known Klein tunneling (i.e., perfect transmission) at normal incidence and the appearance of perfect transmission at oblique incidence due to trigonal warping at energies beyond the linear Dirac regime. We show that this phenomenon arises from the conservation of a generalized pseudospin in the tight-binding model. We expect this effect to be experimentally observable in graphene and other Dirac fermions systems, such as the surface of three-dimensional topological insulators.

The classical Fourier's law fails in extremely small and ultrafast heat conduction even at ordinary temperatures due to strong thermodynamic nonequilibrium effects. In this work, a macroscopic phonon hydrodynamic equation beyond Fourier's law with a relaxation term and nonlocal terms is derived through a perturbation expansion to the phonon Boltzmann equation around a four-moment nonequilibrium solution. The temperature jump and heat flux tangential retardant boundary conditions are developed based on the Maxwell model of the phonon-boundary interaction. Extensive steady-state and transient nanoscale heat transport cases are modeled by the phonon hydrodynamic model, which produces quantitative predictions in good agreement with available phonon Boltzmann equation solutions and experimental results. The phonon hydrodynamic model provides a simple and elegant mathematical description of non-Fourier heat conduction with a clear and intuitive physical picture. The present work will promote deeper understanding and macroscopic modeling of heat transport in extreme states.

We study the effects of the quantum geometric tensor, i.e., the Berry curvature and the Fubini-Study metric, on the steady state of driven-dissipative bosonic lattices. We show that the quantum-Hall-type response of the steady-state wave function in the presence of an external potential gradient depends on all the components of the quantum geometric tensor. Looking at this steady-state Hall response, one can map out the full quantum geometric tensor of a sufficiently flat band in momentum space using a driving field localized in momentum space. We use the two-dimensional Lieb lattice as an example and numerically demonstrate how to measure the quantum geometric tensor.

We study the Landau levels (LLs) of a Weyl semimetal with two adjacent Weyl nodes. We consider different orientations $\eta =\angle (\mathbf{B},{\mathbf{k}}_0)$ of magnetic field $\mathbf{B}$ with respect to ${\mathbf{k}}_0$, the vector of Weyl node splitting. A magnetic field facilitates the tunneling between the nodes, giving rise to a gap in the transverse energy of the zeroth LL. We show how the spectrum is rearranged at different $\eta$ and how this manifests itself in the change of behavior of the differential magnetoconductance $dG\left(B\right)/dB$ of a ballistic $p\text{−}n$ junction. Unlike the single-cone model where Klein tunneling reveals itself in positive $dG\left(B\right)/dB$, in the two-cone case, $G\left(B\right)$ is nonmonotonic with a maximum at $B_c\propto {\mathrm{\Phi }}_0{k}_0^2/ln\left(k_0{l}_E\right)$ for large $k_0{l}_E$, where $l_E=\sqrt{\hslash v/\left|e\right|E}$, with $E$ for the built-in electric field and ${\mathrm{\Phi }}_0$ for the magnetic flux quantum.

A microscopic approach is developed to the scattering of surface states from a nonmagnetic linear defect at a surface with strong spin-orbit interaction. Spin-selective reflection resonances in scattering of Rashba-split surface states by an atomic stripe are theoretically discovered in a proof-of-principle calculation for a model crystal potential. Spin-filtering properties of such linear defects are analyzed within an envelope-function formalism for a perturbed surface based on the Rashba Hamiltonian. The continuous Rashba model is found to be in full accord with the microscopic theory, which reveals the essential physics behind the scattering resonance. The spin-dependent reflection suggests a novel mechanism to manipulate spins on the nanoscale.

The two-dimensional conducting gas (2DCG) that forms at the interface between ${\mathrm{LaAlO}}_3$ (LAO) and ${\mathrm{SrTiO}}_3$ (STO) has been widely studied due to the multitude of in situ tunable phenomena that exist at the interface. Recently it has been shown that nearly every property of the 2DEG that forms at the interface of (111) oriented LAO/STO is strongly anisotropic with respect to the in-plane crystal direction. This in-plane rotational symmetry breaking points to the existence of an electronic nematic phase at the interface that can be modified by an in situ electrostatic back-gate potential. Here we show that the onset temperature of the anisotropy in the longitudinal resistance is $T\approx 22\mathrm{K}$, which does not match up with any known structural transition, and coincides with the onset of anisotropy in the Hall response of the system. Furthermore, below 22 K, charge transport is activated in nature with different activation energies along the two in-plane crystal directions. Such a response implies that the band edges along the two directions are different and provides further evidence of an electronic nematic state at the interface.

Although Fe-based superconductors are correlated electronic systems with multiorbital, previous nuclear magnetic resonance (NMR) measurement suggests that a single spin-fluid model is sufficient to describe its spin behavior. Here, we first observed the breakdown of single spin-fluid model in a heavily hole-doped Fe-based superconductor ${\mathrm{CsFe}}_2{\mathrm{As}}_2$ by site-selective NMR measurement. At high-temperature regime, both Knight shift and nuclear spin-lattice relaxation at ${}^{133}\mathrm{Cs}$ and ${}^{75}\mathrm{As}$ nuclei exhibit distinct temperature-dependent behavior, suggesting the breakdown of the single spin-fluid model in ${\mathrm{CsFe}}_2{\mathrm{As}}_2$. This is ascribed to the coexistence of both localized and itinerant spin degree of freedom at $3d$ orbitals, which is consistent with the orbital-selective Mott phase. With decreasing temperature, the single spin-fluid behavior is recovered below $T^{*}\sim 75$ K due to a coherent state among $3d$ orbitals. The Kondo liquid scenario is proposed to understand the low-temperature coherent state.

The discovery of superconductivity in the two-leg ladder compound ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ has established the 123-type iron chalcogenides as a novel and interesting subgroup of the iron-based superconductor family. However, in this 123 series, ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ is an exceptional member, with a magnetic order and crystalline structure different from all others. Recently, an exciting experiment reported the emergence of superconductivity in ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ at high pressure [J. Ying et al., Phys. Rev. B 95, 241109(R) (2017)]. In this paper, we report a first-principles study of ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$. Our analysis unveils a variety of qualitative differences between ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ and ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$, including in the latter an unexpected chain of transitions with increasing pressure. First, by gradually reducing the tilting angle of iron ladders, the crystalline structure smoothly transforms from $Pnma$ to $Cmcm$ at $\sim 6$ GPa. Second, the system becomes metallic at 10.4 GPa. Third, its unique ambient-pressure Block antiferromagnetic ground state is replaced by the more common stripe (so-called CX-type) antiferromagnetic order at $\sim 12$ GPa, the same magnetic state as the 123-S ladder. This transition is found at a pressure very similar to the experimental superconducting transition. Finally, all magnetic moments vanish at 30 GPa. This reported theoretical diagram of the complete phase evolution is important because of the technical challenges to capture many physical properties in high-pressure experiments. The information obtained in our calculations suggests different characteristics for superconductivity in ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ and ${\mathrm{BaFe}}_2{\mathrm{S}}_3$: in 123-S pairing occurs when magnetic moments vanish, while in 123-Se the transition region from Block- to CX-type magnetism appears to catalyze superconductivity. Finally, an additional superconducting dome above $\sim 30$ GPa is expected to occur.

We analyze the spectrum and eigenstates of a quantum particle in a bipartite two-dimensional tight-binding dice network. In the absence of a dc bias, it hosts a chiral flatband with compact localized eigenstates. In the presence of a dc bias, the energy spectrum consists of a periodic repetition of one-dimensional energy band multiplets, with one member in the multiplet being strictly flat. The corresponding flatband eigenstates cease to be compact, and are localized exponentially perpendicular to the dc field direction, and superexponentially along the dc field direction. The band multiplets are characterized by a topological quantized winding number (Zak phase), which changes at specific values of the varied dc field strength. These changes are induced by gap closings between the flat and dispersive bands, and reflect the number of these closings.