The properties of a phase at finite interactions can be significantly influenced by the underlying dispersion of the noninteracting Hamiltonian. We demonstrate this by studying the repulsive Hubbard model on the two-dimensional Lieb lattice, which has a flat band for vanishing interaction $U$. We perform real-space dynamical mean-field theory calculations at different temperatures and dopings using a continuous-time quantum Monte Carlo impurity solver. Studying the frequency dependence of the self-energy, we find that a nonmagnetic metallic region at finite temperature displays non-Fermi-liquid behavior, which is a concomitant of the flat-band singularity. At half-filling, we also find a magnetically ordered region, where the order parameter varies linearly with the interaction strength, and a strongly correlated Mott insulating phase. The double occupancy decreases sharply for small $U$, highlighting the flat-band contribution. Away from half-filling, we observe the stripe order, i.e., an inhomogeneous spin and charge density wave of finite wavelength, which turns into a sublattice ordering at higher temperatures.

${\mathrm{NaFe}}_{0.5}{\mathrm{Cu}}_{0.5}\mathrm{As}$ represents a rare exception in the metallic iron pnictide family, in which a small insulating gap is opened. Based on first-principles study, we provide a comprehensive theoretical characterization of this insulating compound. The ${\mathrm{Fe}}^{3+}$ spin degree of freedom is quantified as a quasi-one-dimensional (1D) $S=\frac{5}{2}$ Heisenberg model. The itinerant As hole state is downfolded to a $p_{xy}$-orbital hopping model on a square lattice. An orbital-dependent Hund's coupling between the spin and the hole is revealed. Several important material properties are analyzed, including (a) the factors affecting the small $p-d$ charge-transfer gap; (b) the role of extra interchain Fe atoms; and (c) quasi-1D spin excitation in the Fe chains. The experimental manifestations of these properties are discussed.

We report measurements of the magnetoresistance in the charge density wave (CDW) state of rare-earth tritellurides, namely ${\mathrm{TbTe}}_3$ and ${\mathrm{HoTe}}_3$. The magnetic field dependence of magnetoresistance exhibits a temperature dependent crossover between a conventional quadratic law at high $T$ and low $B$ and an unusual linear dependence at low $T$ and high $B$. We present a quite general model to explain the linear magnetoresistance taking into account the strong scattering of quasiparticles on CDW fluctuations in the vicinity of “hot spots” of the Fermi surface (FS) where the FS reconstruction is the strongest.

We have measured the strain, magnetization, and specific heat of the antiferromagnetic (AFM) Kondo semiconductors $\mathrm{Ce}{T}_2\mathrm{A}{\mathrm{l}}_{10}$ ($T=\mathrm{Ru}$ and Os) under uniaxial pressures applied along the orthorhombic axes. We found a linear dependence of $T_{\mathrm{N}}$ on the $b$-axis parameter for both compounds under uniaxial pressure $P\parallel b$ and hydrostatic pressure. This relation indicates that the distance between the Ce-$T$ layers along the $b$ axis is the key structural parameter determining $T_{\mathrm{N}}$. Furthermore, the pressure dependence of the spin-flop transition field indicates that Ce-Ce interchain interactions stabilize the AFM state with the ordered moments pointing to the $c$ axis.

X-ray absorption spectroscopy measured at the $L$ edge of transition metals (TMs) is a powerful element-selective tool providing direct information about the correlation effects in the $3d$ states. The theoretical modeling of the $2p\to 3d$ excitation processes remains to be challenging for contemporary ab initio electronic structure techniques, due to strong core-hole and multiplet effects influencing the spectra. In this work, we present a realization of the method combining the density-functional theory with multiplet ligand field theory, proposed in Haverkort et al. [Phys. Rev. B 85, 165113 (2012)]. In this approach, a single-impurity Anderson model (SIAM) is constructed, with almost all parameters obtained from first principles, and then solved to obtain the spectra. In our implementation, we adopt the language of the dynamical mean-field theory and utilize the local density of states and the hybridization function, projected onto TM $3d$ states, in order to construct the SIAM. The developed computational scheme is applied to calculate the $L$-edge spectra for several TM monoxides. A very good agreement between the theory and experiment is found for all studied systems. The effect of core-hole relaxation, hybridization discretization, possible extensions of the method as well as its limitations are discussed.

The implementation of quantum entangling gates between qubits is essential to achieve scalable quantum computation. Here, we propose a robust scheme to realize an entangling gate for distant solid-state spins via a mechanical oscillator in its thermal equilibrium state. By appropriate Hamiltonian engineering and usage of a protected subspace, we show that the proposed scheme is able to significantly reduce the thermal effect of the mechanical oscillator on the spins. In particular, we demonstrate that a high entangling gate fidelity can be achieved even for a relatively high thermal occupation. Our scheme can thus relax the requirement for ground-state cooling of the mechanical oscillator, and may find applications in scalable quantum information processing in hybrid solid-state architectures.

We present an ab initio study of the role of interference effects in the thermal conductance of single-molecule junctions. To be precise, using a first-principles transport method based on density functional theory, we analyze the coherent phonon transport in single-molecule junctions made of several benzene and oligo(phenylene ethynylene) derivatives. We show that the thermal conductance of these junctions can be tuned via the inclusion of substituents, which induces destructive interference effects and results in a decrease of the thermal conductance with respect to the unmodified molecules. In particular, we demonstrate that these interference effects manifest as antiresonances in the phonon transmission, whose energy positions can be tuned by varying the mass of the substituents. Our work provides clear strategies for the heat management in molecular junctions and, more generally, in nanostructured metal-organic hybrid systems, which are important to determine how these systems can function as efficient energy-conversion devices such as thermoelectric generators and refrigerators.

We demonstrate evidences of electronic transport via topological Dirac surface states in a thin film of strained HgTe. At high perpendicular magnetic fields, we show that the electron transport reaches the quantum Hall regime with vanishing resistance. Furthermore, quantum Hall transport spectroscopy reveals energy splittings of relativistic Landau levels specific to coupled Dirac surface states. This study provides insights in the quantum Hall effect of topological insulator (TI) slabs, in the crossover regime between two- and three-dimensional TIs, and in the relevance of thin TI films to explore circuit functionalities in spintronics and quantum nanoelectronics.

We study the robustness of nonlocal string order in two paradigmatic disordered spin-chain models, a spin-1/2 cluster-Ising and a spin-1 XXZ Heisenberg chain. In the clean case, they both display a transition from antiferromagnetic to string order. Applying a disorder, which preserves the Hamiltonian symmetries, we find that the transition persists in both models. In the disordered cluster-Ising model, we can study the transition analytically—by applying the strongest coupling renormalization group —and numerically—by exploiting integrability to study the antiferromagnetic and string order parameters. We map the model into a quadratic fermion chain, where the transition appears as a change in the number of zero-energy edge modes. We also explore its zero-temperature-singularity behavior and find a transition from a nonsingular to a singular region, at a point that is different from the one separating nonlocal and local ordering. The disordered Heisenberg chain can be treated only numerically: by means of MPS-based simulations, we are able to locate the existence of a transition between antiferromagnetic and string-ordered phase, through the study of order parameters. Finally, we discuss possible connections of our findings with many-body localization.

FeSe, as the simplest Fe-based superconductor, invokes tremendous studies on its electronic and magnetic properties. Among these, inconsistent or even contradictory results are often seen in both theory and experiment, especially concerning the magnetism of FeSe. In this paper, the absence of static magnetic order is directly revealed by the magnetism of Se vacancies in FeSe thin films on ${\mathrm{SrTiO}}_3$ measured with spin-polarized scanning tunneling microscopy on the atomic scale. Symmetry analysis and first-principles calculation further confirmed the nonmagnetic ground state of FeSe. The discrepancy between local-density approximation and generalized-gradient approximation implies that FeSe is near a magnetic critical point.

We study a superconducting artificial atom which is represented by a single Josephson junction or a Josephson junction chain, capacitively coupled to a coherently driven transmission line, and which contains exactly one residual quasiparticle (or less than one quasiparticle per island in a chain). We study the dissipation in the atom induced by the quasiparticle tunneling, taking into account the quasiparticle heating by the drive. We calculate the transmission coefficient in the transmission line for drive frequencies near resonance and show that, when the artificial atom spectrum is nearly harmonic, the intrinsic quality factor of the resonance increases with the drive power. This counterintuitive behavior is due to the energy dependence of the quasiparticle density of states.

It has recently been found that bosonic excitations of ordered media, such as phonons or spinons, can exhibit topologically nontrivial band structures. Of particular interest are magnon and triplon excitations in quantum magnets, as they can easily be manipulated by an applied field. Here, we study triplon excitations in an $S=1/2$ quantum spin ladder and show that they exhibit nontrivial topology, even in the quantum-disordered paramagnetic phase. Our analysis reveals that the paramagnetic phase actually consists of two separate regions with topologically distinct triplon excitations. We demonstrate that the topological transition between these two regions can be tuned by an external magnetic field. The winding number that characterizes the topology of the triplons is derived and evaluated. By bulk-boundary correspondence, we find that the nonzero winding number implies the presence of localized triplon end states. Experimental signatures and possible physical realizations of the topological paramagnetic phase are discussed.

The superconductor ${\mathrm{PdTe}}_2$ was recently classified as a type-II Dirac semimetal, and advocated to be an improved platform for topological superconductivity. Here, we report magnetic and transport measurements conducted to determine the nature of the superconducting phase. Surprisingly, we find that ${\mathrm{PdTe}}_2$ is a type-I superconductor with $T_c=1.64$ K and a critical field ${\mu }_0{H}_c\left(0\right)=13.6$ mT. Our crystals also exhibit the intermediate state as demonstrated by the differential paramagnetic effect. For $H>H_c$ we observe superconductivity of the surface sheath. This calls for a close examination of superconductivity in ${\mathrm{PdTe}}_2$ in view of the presence of topological surface states.

The amorphous structure of the phase change material ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST) has been the object of controversial structural models. By employing first-principles molecular dynamics within density functional theory, we are able to obtain quantitative agreement with experimental structural findings for the topology of glassy GST. To this end, we take full advantage of a thoughtful, well established choice of the exchange-correlation (XC) functional (Becke-Lee-Yang-Parr, BLYP), combined with appropriate options for the nonlocal part in the pseudopotential construction for Ge. Results obtained by using the Perdew-Burke-Ernzerhof (PBE) XC functional and a similar strategy for the Ge pseudopotential constructions are also presented, since they are very valuable and worthy of consideration. The atomic structure of glassy GST is characterized by Ge atoms lying in a predominant tetrahedral network, albeit a non-negligible fraction of Ge atoms are also found in defective octahedra.

We revisit the problem of decoherence of a qubit centrally coupled to an interacting spin environment, here modeled by a quantum compass chain or an extended XY model in a staggered magnetic field. These two models both support distinct spin-liquid phases, adding a new element to the problem. By analyzing their Loschmidt echoes when perturbed by the qubit we find that a fast decoherence of the qubit is conditioned on the presence of propagating quasiparticles which couple to it. Differently from expectations based on earlier works on central spin models, our result implies that the closeness of an environment to a quantum phase transition is neither a sufficient nor a necessary condition for an accelerated decoherence rate of a qubit.

The frustrated one-dimensional quantum magnet ${\mathrm{LiCuSbO}}_4$ is a rare realization of the $J_1-J_2$ spin chain model with an easily accessible saturation field, formerly estimated at 12 T. Exotic multipolar nematic phases were theoretically predicted in such compounds just below the saturation field, but without unambiguous experimental observation so far. In this paper we present extensive experimental research on the compound in a wide temperature (30 mK to 300 K) and field (0–13.3 T) range by muon spin rotation ($\mu \mathrm{SR}$), ${}^7\mathrm{Li}$ nuclear magnetic resonance (NMR), and magnetic susceptibility (SQUID). $\mu \mathrm{SR}$ experiments in zero magnetic field demonstrate the absence of long-range 3D ordering down to 30 mK. Together with former heat capacity data [Dutton et al., Phys. Rev. Lett. 108, 187206 (2012)], magnetic susceptibility measurements suggest a short-range-correlated vector chiral phase in the field range 0–4 T. At the intermediate-field values (5–12 T), the system enters a 3D-ordered spin density wave phase with $0.75{\mu }_B$ per copper site at lowest temperatures (125 mK), estimated by NMR. At still higher field, the magnetization is found to be saturated above 13 T where the spin lattice $T_1^{-1}$ relaxation reveals a spin gap estimated at 3.2(2) K. We narrow down the possibility of observing a multipolar nematic phase to the range 12.5–13 T.

The thermal fluctuations contribution to magnetization and magnetoconductivity of type-II layered superconductors is calculated in the framework of the Lawrence-Doniach model. For numerous high-temperature cuprate superconductors, it was discovered that the magnetization dependence on temperature in a wide range of fields exhibits an intersection point at a temperature slightly below $T_c$. We notice a similar intersection point of the magnetoconductivity curves at the approximate same temperature. The phenomenon is explained by strong (non-Gaussian) thermal fluctuations with interactions treated using a self-consistent theory. All higher Landau levels should be included. Dimensionality of the fluctuations is defined, and the two- to three-dimensional crossover is the key for the existence of intersection points.

Magneto-optical (MO) response of the noble metals Cu, Ag, and Au in the joint experimental and ab initio theoretical study is reported. The magneto-optical polar Kerr effect (MOKE) spectra of the noble-metal films were measured with the high sensitivity in the applied magnetic field of 1.5 T over the photon energy range 0.74–5.8 eV. Complete set of the optical conductivity tensor elements was determined precisely from the MOKE and the optical spectra measured at the same energy points. The importance of the off-diagonal intraband Drude-type transitions is demonstrated explicitly for each noble metal and found to be a substantial contribution to the observed spectra. It is shown that the first-principles calculations using the spin-polarized fully relativistic Dirac linear-muffin-tin-orbital method with the inclusion of correlation effects by GGA+$U$ approach reproduce well the experimental spectra and allow to explain the microscopic origin of the noble metals' magneto-optical response in terms of interband transitions. Although the energy band structures of Cu, Ag, and Au are very similar, there are some distinctive differences in bandwidths and the energy positions of the bands (especially in $X$ and $L$ symmetry points), mainly due to different spin-orbit splitting and differences in the spatial extent of $3d, 4d$, and $5d$ valence wave functions of noble metals. It was found that the small differences in the band positions lead to significant differences in the MO properties of three noble metals. Although the spin-orbit interaction in Au is about six times larger than in Cu, and approximately two times larger than in Ag, the absolute value of Kerr rotation in Au is of the same magnitude as in Cu and one order of magnitude smaller as compared to Ag. The sharp Kerr effect spectral peak in Ag is not due to the electronic interband transitions, but rather to the plasma-edge splitting. The band-by-band decomposition of the Cu, Ag, and Au MO spectra is presented and the interband transitions responsible for the prominent structures in the spectra are identified. It has been found that main magneto-optical activity of noble metals in external magnetic field originates from interband transitions at well-defined small-volume regions of Brillouin zone located near the “neck” and “belly” of the Fermi surface.