We have performed an angle-resolved photoemission study of the nodal quasiparticle spectra of the high-$T_c$ cuprate trilayer ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{10+\delta }$ ($T_c\sim 110$ K). The spectral weight $Z$ of the nodal quasiparticle increases with decreasing temperature across $T_c$. Such a temperature dependence is qualitatively similar to that of the coherence peak intensity in the antinodal region of various high-$T_c$ cuprates, although the nodal spectral weight remains finite and large above $T_c$. We attribute this observation to the reduction of electron correlation strength in going from the normal metallic state to the superconducting state, a characteristic behavior of a superconductor with strong electron correlation.

We predict that guiding center (GC) diffusion yields a linear and nonsaturating (transverse) magnetoresistance in 3D metals. Our theory is semiclassical and applies in the regime where the transport time is much greater than the cyclotron period and for weak disorder potentials which are slowly varying on a length scale much greater than the cyclotron radius. Under these conditions, orbits with small momenta along magnetic field $B$ are squeezed and dominate the transverse conductivity. When disorder potentials are stronger than the Debye frequency, linear magnetoresistance is predicted to survive up to room temperature and beyond. We argue that magnetoresistance from GC diffusion explains the recently observed giant linear magnetoresistance in 3D Dirac materials.

We directly measure the electronic structure of twisted graphene/$\mathrm{Mo}{\mathrm{S}}_2$ van der Waals heterostructures, in which both graphene and $\mathrm{Mo}{\mathrm{S}}_2$ are monolayers. We use cathode lens microscopy and microprobe angle-resolved photoemission spectroscopy measurements to image the surface, determine twist angle, and map the electronic structure of these artificial heterostructures. For monolayer graphene on monolayer $\mathrm{Mo}{\mathrm{S}}_2$, the resulting band structure reveals the absence of hybridization between the graphene and $\mathrm{Mo}{\mathrm{S}}_2$ electronic states. Further, the graphene-derived electronic structure in the heterostructures remains essentially intact, irrespective of the twist angle between the two materials. In contrast, however, the electronic structure associated with the $\mathrm{Mo}{\mathrm{S}}_2$ layer is found to be twist-angle dependent; in particular, the relative difference in the energy of the valence band maximum at $\overline{\mathrm{\Gamma }}$ and $\overline{K}$ of the $\mathrm{Mo}{\mathrm{S}}_2$ layer varies from approximately 0 to 0.2 eV. Our results suggest that monolayer $\mathrm{Mo}{\mathrm{S}}_2$ within the heterostructure becomes predominantly an indirect band-gap system for all twist angles except in the proximity of ${30}^{\circ }$. This result enables potential band-gap engineering in van der Waals heterostructures comprised of monolayer structures.

We develop a numerical algorithm for identifying approximately conserved quantities in models perturbed away from integrability. In the long-time regime, these quantities fully determine correlation functions of local observables. Applying the algorithm to the perturbed XXZ model, we find that the main effect of perturbation consists in expanding the support of conserved quantities. This expansion follows quadratic dependence on the strength of perturbation. The latter result, together with correlation functions of conserved quantities obtained from the memory function analysis, confirms the feasibility of the perturbation theory.

We discuss topological and electronic properties of thin films of the topological insulator ${\mathrm{Bi}}_{\text{2}}{\mathrm{Se}}_{\text{3}}$ employing the $GW$ method. We obtain a topologically trivial phase for films of one to six quintuple layers thickness, whereas a quantum spin Hall phase is observed in neither of these films. This challenges recent contradictory reports on quantum spin Hall phases in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films from both theory and experiment. Moreover, we argue that one cannot conclude the topological phase from the band structures measured in those studies. The reliability of our results is supported by the excellent agreement of all calculated spectral features with photoemission experiments, i.e., the band gaps and the energy and slope of the Dirac state.

We investigate the effectiveness of a microwave cavity as a mediator of interactions between two resonant exchange (RX) qubits in semiconductor quantum dots (QDs) over long distances, limited only by the extension of the cavity. Our interaction model includes the orthonormalized Wannier orbitals constructed from Fock-Darwin states under the assumption of a harmonic QD confinement potential. We calculate the qubit-cavity coupling strength in a Jaynes-Cummings Hamiltonian and find that dipole transitions between two states with an asymmetric charge configuration constitute the relevant RX qubit-cavity coupling mechanism. The effective coupling between two RX qubits in a shared cavity yields a universal two-qubit iswap gate with gate times on the order of nanoseconds over distances on the order of up to a millimeter.

We study the formation of modulated martensites in ferromagnetic shape memory alloys by a mathematical model originating from the nanotwinning concept. The results show that the incommensurateness, systematically observed in experiments for the modulated phases, may be understood as a precursor effect of the intermartensitic transitions, and its appearance does not contradict the nanotwinning concept itself. The model sufficiently explains the different levels of incommensurateness reported from different experimental observations for the 14-layered and 10-layered martensites of the Ni-Mn-Ga alloy and outlines the mechanism of formation of faults in the stacking sequences of these materials.

Layered organic superconductors of the BEDT family are model systems for understanding the interplay of the Mott transition with superconductivity, magnetic order, and frustration, ingredients that are essential to understand superconductivity also in the cuprate high-temperature superconductors. Recent experimental studies on a hole-doped version of the organic compounds reveals an enhancement of superconductivity and a rapid crossover between two different conducting phases above the superconducting dome. One of these phases is a Fermi liquid, the other not. Using plaquette cellular dynamical mean field theory with state-of-the-art continuous-time quantum Monte Carlo calculations, we study this problem with the two-dimensional Hubbard model on the anisotropic triangular lattice. Phase diagrams as a function of temperature $T$ and interaction strength $U/t$ are obtained for anisotropy parameters $t^{\prime }=0.4t,t^{\prime }=0.8t$ and for various fillings. As in the case of the cuprates, we find, at finite doping, a first-order transition between two normal-state phases. One of theses phases has a pseudogap while the other does not. At temperatures above the critical point of the first-order transition, there is a Widom line where crossovers occur. The maximum (optimal) superconducting critical temperature $T_c^m$ at finite doping is enhanced by about 25% compared with its maximum at half filling and the range of $U/t$ where superconductivity appears is greatly extended. These results are in broad agreement with experiment. Also, increasing frustration (larger $t^{\prime }/t)$ significantly reduces magnetic ordering, as expected. This suggests that for compounds with intermediate to high frustration, very light doping should reveal the influence of the first-order transition and associated crossovers. These crossovers could possibly be even visible in the superconducting phase through subtle signatures. We also predict that destroying the superconducting phase by a magnetic field should reveal the first-order transition between metal and pseudogap. Finally, we predict that electron doping should also lead to an increased range of $U/t$ for superconductivity but with a reduced maximum $T_c$. This work also clearly shows that the superconducting dome in organic superconductors is tied to the Mott transition and its continuation as a transition separating pseudogap phase from correlated metal in doped compounds, as in the cuprates. Contrary to heavy fermions for example, the maximum $T_c$ is definitely not attached to an antiferromagnetic quantum critical point. That can also be verified experimentally.

We explore theoretically the magnetoresistivity of three-dimensional Weyl and Dirac semimetals in transversal magnetic fields within two alternative models of disorder: (i) short-range impurities and (ii) charged (Coulomb) impurities. Impurity scattering is treated using the self-consistent Born approximation. We find that an unusual broadening of Landau levels leads to a variety of regimes of the resistivity scaling in the temperature-magnetic field plane. In particular, the magnetoresistance is nonmonotonous for the white-noise disorder model. For $H\to 0$ the magnetoresistance for short-range impurities vanishes in a nonanalytic way as $H^{1/3}$. In the limits of strongest magnetic fields $H$, the magnetoresistivity vanishes as $1/H$ for pointlike impurities, while it is linear and positive in the model with Coulomb impurities.

The application of a magnetic field transverse to the easy axis, Ising direction in the quasi-two-dimensional kagome staircase magnet, ${\mathrm{Co}}_3{\mathrm{V}}_2{\mathrm{O}}_8$, induces three quantum phase transitions at low temperatures, ultimately producing a novel high field polarized state, with two distinct sublattices. New time-of-flight neutron scattering techniques, accompanied by large angular access, high magnetic field infrastructure allow the mapping of a sequence of ferromagnetic and incommensurate phases and their accompanying spin excitations. At least one of the transitions to incommensurate phases at ${\mu }_0{H}_{c1}\sim 6.25$ T and ${\mu }_0{H}_{c2}\sim 7$ T is discontinuous, while the final quantum critical point at ${\mu }_0{H}_{c3}\sim 13$ T is continuous.

We present an accurate ab initio tight-binding Hamiltonian for the transition metal dichalcogenides, ${\mathrm{MoS}}_2, {\mathrm{MoSe}}_2, {\mathrm{WS}}_2, {\mathrm{WSe}}_2$, with a minimal basis (the $d$ orbitals for the metal atoms and $p$ orbitals for the chalcogen atoms) based on a transformation of the Kohn-Sham density functional theory Hamiltonian to a basis of maximally localized Wannier functions. The truncated tight-binding Hamiltonian, with only on-site, first, and partial second neighbor interactions, including spin-orbit coupling, provides a simple physical picture and the symmetry of the main band-structure features. Interlayer interactions between adjacent layers are modeled by transferable hopping terms between the chalcogen $p$ orbitals. The full-range tight-binding Hamiltonian can be reduced to hybrid-orbital $\mathbf{k}·\mathbf{p}$ effective Hamiltonians near the band extrema that capture important low-energy excitations. These ab initio Hamiltonians can serve as the starting point for applications to interacting many-body physics including optical transitions and Berry curvature of bands, of which we give some examples.

Synchrotron reflection x-ray thermal diffuse scattering (TDS) measurements, rather than previously reported transmission TDS, are carried out at room temperature and analyzed using a formalism based upon second-order interatomic force constants and long-range Coulomb interactions to obtain quantitative determinations of MgO phonon dispersion relations $\hslash {\omega }_j$(q), phonon densities of states $g$($\hslash \omega$), and isochoric temperature-dependent vibrational heat capacities $c_v\left(T\right)$. We use MgO as a model system for investigating reflection TDS due to its harmonic behavior as well as its mechanical and dynamic stability. Resulting phonon dispersion relations and densities of states are found to be in good agreement with independent reports from inelastic neutron and x-ray scattering experiments. Temperature-dependent isochoric heat capacities $c_v\left(T\right)$, computed within the harmonic approximation from $\hslash {\omega }_j$(q) values, increase with temperature from $0.4\times {10}^{-4}\mathrm{eV}/\mathrm{atom}\mathrm{K}$ at 100 K to $1.4\times {10}^{-4}\mathrm{eV}/\mathrm{atom}\mathrm{K}$ at 200 K and $1.9\times {10}^{-4}\mathrm{eV}/\mathrm{atom}\mathrm{K}$ at 300 K, in excellent agreement with isobaric heat capacity values $c_p\left(T\right)$ between 4 and 300 K. We anticipate that the experimental approach developed here will be valuable for determining vibrational properties of heteroepitaxial thin films since the use of grazing-incidence ($\theta \lesssim {\theta }_c$, where ${\theta }_c$ is the density-dependent critical angle) allows selective tuning of x-ray penetration depths to $\lesssim 10\mathrm{nm}$.

Even if a noninteracting system has zero Berry curvature everywhere in the Brillouin zone, it is possible to introduce interactions that stabilize a fractional Chern insulator. These interactions necessarily break time-reversal symmetry (either spontaneously or explicitly) and have the effect of altering the underlying band structure. We outline a number of ways in which this may be achieved and show how similar interactions may also be used to create a (time-reversal-symmetric) fractional topological insulator. While our approach is rigorous in the limit of long-range interactions, we show numerically that even for short-range interactions a fractional Chern insulator can be stabilized in a band with zero Berry curvature.

Topological insulators provide a new platform for spintronics due to the spin texture of the surface states that are topologically robust against elastic backscattering. Here we report on an investigation of the measured voltage obtained from efforts to electrically probe spin-momentum locking in the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ using ferromagnetic contacts. Upon inverting the magnetization of the ferromagnetic contacts, we find a reversal of the measured voltage. Extensive analysis of the bias and temperature dependence of this voltage was done, considering the orientation of the magnetization relative to the current. Our findings indicate that the measured voltage can arise due to fringe-field-induced Hall voltages, different from current-induced spin polarization of the surface state charge carriers, as reported recently. Understanding the nontrivial origin of the measured voltage is important for realizing spintronic devices with topological insulators.

We study the fate of superconductivity in the vicinity of a class of metallic quantum critical points obtained by coupling a Fermi surface to a critical boson. In such systems there is a competition between the enhanced pairing tendency due to the presence of long-range attractive interactions near criticality, and the suppression of superconductivity due to the destruction of the Landau quasiparticles. We show that there are regimes in which these two effects offset one another, resulting in a non-Fermi liquid fixed point with finite, scale invariant, BCS coupling. While these interactions lead to substantial superconducting fluctuations, they do not drive the system into a superconducting ground state. The metallic quantum critical fixed points are connected to the superconducting regime by a continuous phase transition. These results are established using a controlled expansion in the deviation from $d=3$ spatial dimensions, as well as in a large number $N$ of internal flavors. We discuss the possible relevance of our findings to the phenomenon of superconducting domes condensing out of a non-Fermi liquid normal state near quantum critical points.

In this paper we study the critical behavior of an $N$-component ${\mathbf{\varphi }}^4$ model in hyperbolic space, which serves as a model of uniform frustration. We find that this model exhibits a second-order phase transition with an unusual magnetization texture that results from the lack of global parallelism in hyperbolic space. Angular defects occur on length scales comparable to the radius of curvature. This phase transition is governed by a strong curvature fixed point that obeys scaling below the upper critical dimension $d_{uc}=4$. The exponents of this fixed point are given by the leading order terms of the $1/N$ expansion. In distinction to flat space no order $1/N$ corrections occur. We conclude that the description of many-particle systems in hyperbolic space is a promising avenue to investigate uniform frustration and nontrivial critical behavior within one theoretical approach.

We reexamine the experimental results for the magnetic response function ${\chi }^{\prime \prime }(\mathbf{q},E,T)$ for $\mathbf{q}$ around the antiferromagnetic vectors $\mathbf{Q}$, in the quantum-critical region, obtained by inelastic neutron scattering, on an Fe-based superconductor and on a heavy-fermion compound. The motivation is to compare the results with a recent theory, which shows that the fluctuations in a generic antiferromagnetic model for itinerant fermions map to those in the universality class of the dissipative quantum-$\mathit{XY}$ model. The quantum-critical fluctuations in this model, in a range of parameters, are given by the correlations of spatial and temporal topological defects. The theory predicts a ${\chi }^{\prime \prime }(\mathbf{q},E,T)$ (i) which is a separable function of $\left(\mathbf{q}-\mathbf{Q}\right)$ and of $(E,T)$, (ii) at criticality, the energy-dependent part is $\propto tanh(E/2T)$ below a cutoff energy, (iii) the correlation time departs from its infinite value at criticality on the disordered side by an essential singularity, and (iv) the correlation length depends logarithmically on the correlation time, so that the dynamical critical exponent $z$ is $\infty$. The limited existing experimental results are found to be consistent with the first two unusual predictions from which the linear dependence of the resistivity on $T$ and the $TlnT$ dependence of the entropy also follow. More experiments are suggested, especially to test the theory of variations on the correlation time and length on the departure from criticality.

Motivated by recent experimental progress in preparing encapsulated graphene sheets with ultrahigh mobilities up to room temperature, we present a theoretical study of dc transport in doped graphene in the hydrodynamic regime. By using the continuity and Navier-Stokes equations, we demonstrate analytically that measurements of nonlocal resistances in multiterminal Hall bar devices can be used to extract the hydrodynamic shear viscosity of the two-dimensional (2D) electron liquid in graphene. We also discuss how to probe the viscosity-dominated hydrodynamic transport regime by scanning probe potentiometry and magnetometry. Our approach enables measurements of the viscosity of any 2D electron liquid in the hydrodynamic transport regime.

We analyze the process of thermalization, dynamics, and the eigenstate thermalization hypothesis (ETH) for the single-impurity Anderson model, focusing on the Kondo regime. For this we construct the complete eigenbasis of the Hamiltonian using the numerical renormalization group (NRG) method in the language of the matrix product states. It is a peculiarity of the NRG that while the Wilson chain is supposed to describe a macroscopic bath, very few single-particle excitations already suffice to essentially thermalize the impurity system at finite temperature, which amounts to having added a macroscopic amount of energy. Thus, given an initial state of the system such as the ground state together with microscopic excitations, we calculate the spectral function of the quantum impurity using the microcanonical and diagonal ensembles. These spectral functions are compared to the time-averaged spectral function obtained by time evolving the initial state according to the full Hamiltonian, and to the spectral function calculated using the thermal density matrix. By adding or removing particles at a certain Wilson energy shell on top of the ground state, we find qualitative agreement between the resulting spectral functions calculated for different ensembles. This indicates that the system thermalizes in the long-time limit, and can be described by an appropriate statistical-mechanical ensemble. Moreover, by calculating static quantities such as the impurity spectral density at the Fermi level as well as the dot occupancy for energy eigenstates relevant for microcanonical ensemble, we find good support for the ETH. The ultimate mechanism responsible for this effective thermalization within the NRG can be identified as Anderson orthogonality: the more charge that needs to flow to or from infinity after applying a local excitation within the Wilson chain, the more the system looks thermal afterwards at an increased temperature. For the same reason, however, thermalization fails if charge rearrangement after the excitation remains mostly local. In these cases, the different statistical ensembles lead to different results. Their behavior needs to be understood as a microscopic quantum quench only.

${\mathrm{Fe}}_3{\mathrm{PO}}_4{\mathrm{O}}_3$ forms a noncentrosymmetric lattice structure (space group $R3m$) comprising triangular motifs of ${\mathrm{Fe}}^{3+}$ coupled by strong antiferromagnetic interactions ($|{\mathrm{\Theta }}_{CW}|>900$ K). Neutron diffraction from polycrystalline samples shows that strong frustration eventually gives way to an ordered helical incommensurate structure below $T_N$ = 163 K, with the helical axis in the hexagonal $ab$ plane and a modulation length to $\sim 86$ Å. The magnetic structure consists of an unusual needlelike correlation volume that extends past 900 Å along the hexagonal $c$ axis but is limited to $\sim 70$ Å in the $ab$ plane, despite the three-dimensional nature of the magnetic sublattice topology. The small in-plane correlation length, which persists to at least $T=T_N/40$, indicates a robust blocking of long-range order of the helical magnetic structure, and therefore stable domain walls, or other defect spin textures, must be abundant in ${\mathrm{Fe}}_3{\mathrm{PO}}_4{\mathrm{O}}_3$. Temperature-dependent neutron powder diffraction reveals small negative thermal expansion below $T_N$. No change in lattice symmetry is observed on cooling through $T_N$, as revealed by high-resolution synchrotron x-ray diffraction. The previously reported reduced moment of the ${\mathrm{Fe}}^{3+}$ ions ($S=5/2$), $\mu \sim 4.2{\mu }_B$, is confirmed here through magnetization studies of a magnetically diluted solid solution series of compounds, ${\mathrm{Fe}}_{(3-x)}{\mathrm{Ga}}_x{\mathrm{PO}}_4{\mathrm{O}}_3$, and is consistent with the refined magnetic moment from neutron diffraction 4.14(2) ${\mu }_B$. We attribute the reduced moment to a modified spin density distribution arising from ligand charge transfer in this insulating oxide.