We study a model ${\mathrm{SrTiO}}_3$ interface in which conduction $t_{2g}$ electrons couple to the ferroelectric (FE) phonon mode. We treat the FE mode within a self-consistent phonon theory that captures its quantum critical behavior and show that proximity to the quantum critical point leads to universal tails in the electron density of the form $n\left(z\right)\sim {(\lambda +z)}^{-2}$, where $\lambda \sim T^{2-d/\mathfrak{z}}$, with $d=3$ the dimensionality and $\mathfrak{z}=1$ the dynamical critical exponent. Implications for the metal-insulator transition at low electron density are discussed.

We have performed inelastic neutron scattering measurements on powder samples of the quasicrystal approximant, ${\mathrm{TbCd}}_6$, grown using isotopically enriched ${}^{112}\mathrm{Cd}$. Both quasielastic scattering and distinct inelastic excitations were observed below 3 meV. The intensity of the quasielastic scattering measured in the paramagnetic phase diverges as $T_{\mathrm{N}}\sim 22$ K is approached from above. The inelastic excitations, and their evolution with temperature, are well characterized by the leading term, $B_2^0{O}_2^0$, of the crystal electric field (CEF) level scheme for local pentagonal symmetry for the rare-earth ions [S. Jazbec et al., Phys. Rev. B 93, 054208 (2016)] indicating that the Tb moment is directed primarily along the unique local pseudofivefold axis of the Tsai-type clusters. We also find good agreement between the inverse susceptibility determined from magnetization measurements using a magnetically diluted ${\mathrm{Tb}}_{0.05}{\mathrm{Y}}_{0.95}{\mathrm{Cd}}_6$ sample and that calculated using the CEF level scheme determined from the neutron measurements.

The low-energy magnetic excitation spectrum of the Heisenberg antiferromagnetic $S=1/2$ chain system ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ with Ni and Ca impurities is studied by neutron spectroscopy. In all cases, a defect-induced spectral pseudogap is observed and shown to scale proportionately to the number of scattering centers in the spin chains.

We demonstrate that an all-antiferromagnetic tunnel junction with current perpendicular to the plane geometry can be used as an efficient spintronic device with potential high-frequency operation. By using state-of-the-art density functional theory combined with quantum transport, we show that the Néel vector of the electrodes can be manipulated by spin-transfer torque. This is staggered over the two different magnetic sublattices and can generate dynamics and switching. At the same time the different magnetization states of the junction can be read by standard tunneling magnetoresistance. Calculations are performed for $\mathrm{CuMnAs}\left|\mathrm{GaP}\right|\mathrm{CuMnAs}$ junctions with different surface terminations between the antiferromagnetic CuMnAs electrodes and the insulating GaP spacer. We find that the torque remains staggered regardless of the termination, while the magnetoresistance depends on the microscopic details of the interface.

We investigate the spin-orbit coupling (SOC) effects in $\alpha$- and $\kappa$-phase BEDT-TTF and BEDT-TSF organic salts. Contrary to the assumption that SOC in organics is negligible due to light C, S, and H atoms, we show the relevance of such an interaction in a few representative cases. In the weakly correlated regime, SOC manifests primarily in the opening of energy gaps at degenerate band touching points. This effect becomes especially important for Dirac semimetals such as $\alpha -{\left(\mathrm{ET}\right)}_2{\mathrm{I}}_3$. Furthermore, in the magnetic insulating phase, SOC results in additional anisotropic exchange interactions, which provide a compelling explanation for the puzzling field-induced behavior of the quantum spin-liquid candidate $\kappa -{\left(\mathrm{ET}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$. We conclude by discussing the importance of SOC for the description of low-energy properties in organics.

Density-functional calculations of lattice dynamics and high-resolution synchrotron powder diffraction uncover antiferroelectric distortion in the kagome francisite ${\mathrm{Cu}}_3\mathrm{Bi}{\left({\mathrm{SeO}}_3\right)}_2{\mathrm{O}}_2\mathrm{Cl}$ below 115 K. Its Br-containing analog is stable in the room-temperature crystal structure down to at least 10 K, although the Br compound is on the verge of a similar antiferroelectric instability and reveals local displacements of Cu and Br atoms. The I-containing compound is stable in its room-temperature structure according to density-functional calculations. We show that the distortion involves cooperative displacements of Cu and Cl atoms, and originates from the optimization of interatomic distances for weakly bonded halogen atoms. The distortion introduces a tangible deformation of the kagome spin lattice and may be responsible for the reduced net magnetization of the Cl compound compared to the Br one. The polar structure of ${\mathrm{Cu}}_3\mathrm{Bi}{\left({\mathrm{SeO}}_3\right)}_2{\mathrm{O}}_2\mathrm{Cl}$ is only slightly higher in energy than the nonpolar antiferroelectric structure, but no convincing evidence of its formation could be obtained.

We define two classes of colorings that have odd or even chirality on hexagonal lattices. This parity is an invariant in the dynamics of all loops, and explains why standard Monte Carlo algorithms are nonergodic. We argue that adding the motion of “stranded” loops allows for parity changes. By implementing this algorithm, we show that the even and odd classes have the same entropy. In general, they do not have the same number of states, except for the special geometry of long strips, where a $Z_2$ symmetry between even and odd states occurs in the thermodynamic limit.

The effects of uniaxial strain and hydrostatic pressure on ${\mathrm{Hg}}_{0.83}{\mathrm{Re}}_{0.18}{\mathrm{Ba}}_2{\mathrm{Ca}}_{2.4}{\mathrm{Cu}}_{3.6}{\mathrm{O}}_{14}[{\mathrm{Hg}}_{0.83}\left({\mathrm{Re}}_{0.18}\right)\text{−}1223]$ were investigated by ac magnetic measurements under stress corresponding to a pressure of 20 GPa at maximum. According to a previous thermal study based on the Ehrenfest relation, in-plane contraction should increase the superconducting transition temperature $T_{\mathrm{c}}$, whereas out-of-plane contraction should decrease $T_{\mathrm{c}}$. This suggests that the increase in $T_{\mathrm{c}}$ under hydrostatic-pressure contraction must be smaller than that under in-plane contraction. However, the present uniaxial-strain experiments revealed enhancement of $T_{\mathrm{c}}$ under both in-plane and out-of-plane contraction, and the largest enhancement was observed under hydrostatic-pressure contraction. According to a band calculation, all contraction styles induce hole doping from the HgO blocks to the ${\mathrm{CuO}}_2$ blocks, and hydrostatic-pressure contraction yields the largest hole doping among three contractions. This behavior explains well a series of changes in $T_{\mathrm{c}}$ in the stress region of below 8 GPa. More specifically, under hydrostatic-pressure contraction, $T_{\mathrm{c}}$ exhibited an increase, a decrease, and another increase with increasing pressure, and this multistep change is similar to that observed in Bi-2223-type cuprate superconductors, suggesting that it is necessary to distinguish the effect of strain on the middle ${\mathrm{CuO}}_2$ plane in the three-${\mathrm{CuO}}_2$-plane package from that on the outer planes.

Undoped iron superconductors accommodate $n=6$ electrons in five $d$ orbitals. Experimental and theoretical evidence shows that the strength of correlations increases with hole doping, as the electronic filling approaches half filling with $n=5$ electrons. This evidence delineates a scenario in which the parent compound of iron superconductors is the half-filled system, in analogy to cuprate superconductors. In cuprates the superconductivity can be induced upon electron or hole doping. In this work we propose to search for high-$T_c$ superconductivity and strong correlations in chromium pnictides and chalcogenides with $n<5$ electrons. By means of ab initio slave-spin and multiorbital random-phase-approximation calculations we analyze the strength of the correlations and the superconducting and magnetic instabilities in these systems with the main focus on LaCrAsO. We find that electron-doped LaCrAsO is a strongly correlated system with competing magnetic interactions, with $(\pi ,\pi )$ antiferromagnetism and nodal $d$-wave pairing being the most plausible magnetic and superconducting instabilities, respectively.

The correlation functions related to topological phase transitions in inversion-symmetric lattice models described by $2\times 2$ Dirac Hamiltonians are discussed. In one dimension, the correlation function measures the charge-polarization correlation between Wannier states at different positions, while in two dimensions it measures the itinerant-circulation correlation between Wannier states. The correlation function is nonzero in both the topologically trivial and nontrivial states, and allows us to extract a correlation length that diverges at topological phase transitions. The correlation length and the curvature function that defines the topological invariants are shown to have universal critical exponents, allowing the notion of universality classes to be introduced. Particularly in two dimensions, the universality class is determined by the orbital symmetry of the Dirac model. The scaling laws that constrain the critical exponents are revealed, and are predicted to be satisfied even in interacting systems, as demonstrated in an interacting topological Kondo insulator.

Understanding the effect of pinning on the vortex dynamics in superconductors is a key factor towards controlling critical current values. Large-scale simulations of vortex dynamics can provide a rational approach to achieve this goal. Here, we use the time-dependent Ginzburg-Landau equations to study thin superconducting films with artificially created pinning centers arranged periodically in hexagonal lattices. We calculate the critical current density for various geometries of the pinning centers—varying their size, strength, and density. Furthermore, we shed light upon the influence of pattern distortion on the magnetic-field-dependent critical current. We compare our result directly with available experimental measurements on patterned molybdenum-germanium films, obtaining good agreement. Our results give important systematic insights into the mechanisms of pinning in these artificial pinning landscapes and open a path for tailoring superconducting films with desired critical current behavior.

The electronic transport properties of a monolayer graphene device have been studied before and after the deposition of a dilute coating of tungsten adatoms on the surface. For coverages up to 2.5% of a monolayer, we find tungsten adatoms simultaneously donate electrons to graphene and reduce the carrier mobility, impacting the zero- and finite-field transport properties. Two independent transport analyses suggest the adatoms lie nearly 1 nm above the surface. The presence of adatoms is also seen to impact the low-field magnetoresistance, altering the signatures of weak localization.

In this paper, we show that a single finger-gate capacitor (FGC) can generate pure spin pumping in a quantum channel (QC). Two dynamic fields, ac spin-orbit interaction and ac potential energy, both induced by the FGC onto the QC, are the agents driving the spin pumping. Smooth spatial profiles of the two ac fields are taken into account both perturbatively and full numerically for the nonadiabatic spin pumping. Our perturbative approach reveals that the spin-pumping mechanism is resonant sideband processes associated with simultaneous coupling of the two ac fields with traversing carriers. Full sideband-process treatment is carried out numerically by a time-dependent scattering matrix method. The same spin-pumping mechanism holds also for the case of a single finger-gated QC, albeit with smaller pumping amplitudes.

The electron-photon interaction in two-dimensional materials obeys the rule of “electron valley–photon polarization” correspondence. At the quantum level, such correspondence can be utilized to entangle valleys and polarizations and attain the transfer of quantum states (or information) between valley and photon qubits. Our paper presents a theoretical study of the interaction between the two types of qubits and the resultant quantum state transfer. A generic setup is introduced, which involves optical cavities enhancing the electron-photon interaction as well as facilitating both the entanglement and unentanglement between valleys and polarizations required by the transfer. The quantum system considered consists of electrons, optically excited trions, and cavity photons, with photons moving in and out of the system. A wave equation based analysis is performed, and analytical expressions are derived for the two important figures of merits that characterize the transfer, namely, yield and fidelity, allowing for the investigation of their dependences on various qubit and cavity parameters. A numerical study of the yield and fidelity has also been carried out. Overall, this paper shows promising characteristics in the valley-photon state transfer, with the conclusion that the valley-polarization correspondence can be exploited to achieve the transfer with good yield and high fidelity.

Control of dopants in silicon remains crucial to tailoring the properties of electronic materials for integrated circuits. Silicon is also finding new applications in coherent quantum devices, as a magnetically quiet environment for impurity orbitals. The ionization energies and shapes of the dopant orbitals depend on the surfaces and interfaces with which they interact. The location of the dopant and local environment effects will therefore determine the functionality of both future quantum information processors and next-generation semiconductor devices. Here we match observed dopant wave functions from scanning tunneling microscopy (STM) to images simulated from first-principles density functional theory (DFT) calculations and precisely determine the substitutional sites of neutral As dopants between 5 and 15 Å below the Si(001):H surface. We gain a full understanding of the interaction of the donor state with the surface and the transition between the bulk dopant and the dopants in the surface layer.

Excitonic effects play a particularly important role in the optoelectronic behavior of two-dimensional (2D) semiconductors. To facilitate the interpretation of experimental photoabsorption and photoluminescence spectra we provide statistically exact diffusion quantum Monte Carlo binding-energy data for Mott-Wannier models of excitons, trions, and biexcitons in 2D semiconductors. We also provide contact pair densities to allow a description of contact (exchange) interactions between charge carriers using first-order perturbation theory. Our data indicate that the binding energy of a trion is generally larger than that of a biexciton in 2D semiconductors. We provide interpolation formulas giving the binding energy and contact density of 2D semiconductors as functions of the electron and hole effective masses and the in-plane polarizability.

We report the first measurement of 1/f type noise associated with electronic spin transport, using single layer graphene as a prototypical material with a large and tunable Hooge parameter. We identify the presence of two contributions to the measured spin-dependent noise: contact polarization noise from the ferromagnetic electrodes, which can be filtered out using the cross-correlation method, and the noise originated from the spin relaxation processes. The noise magnitude for spin and charge transport differs by three orders of magnitude, implying different scattering mechanisms for the 1/f fluctuations in the charge and spin transport processes. A modulation of the spin-dependent noise magnitude by changing the spin relaxation length and time indicates that the spin-flip processes dominate the spin-dependent noise.

We use soft x-ray photoemission spectroscopy (SXPES) to investigate Ce $4f$ electronic states of a new ${\mathrm{BiS}}_2$ layered superconductor ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$, for polycrystalline and single-crystal samples. The Ce $3d$ spectrum of the single crystal of nominal composition $x=0.7$ has no $f^0$ component and the spectral shape closely resembles the ones observed for Ce trivalent insulating compounds, strongly implying that the CeO layer is still in an insulating state even after the F doping. The Ce $3d-4f$ resonant SXPES for both polycrystalline and single-crystal samples shows that the prominent peak is located around 1 eV below the Fermi level $\left(E_{\mathrm{F}}\right)$ with negligible spectral intensity at $E_{\mathrm{F}}$. The F-concentration dependence of the valence band spectra for single crystals shows the increases of the degeneracy in energy levels and of the interaction between Ce $4f$ and S $3p$ states. These results give insight into the nature of the ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x$ layer and the microscopic coexistence of magnetism and superconductivity in ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$.

Interaction-driven topological phase transitions in Dirac semimetals are investigated by means of large-scale quantum Monte Carlo simulations. The interaction among Dirac fermions is introduced by coupling them to Ising spins that realize the quantum dynamics of the two-dimensional transverse field Ising model. The ground-state phase diagram, in which the tuning parameters are the transverse field and the coupling between fermion and Ising spins, is determined. At weak and intermediate coupling, a second-order Ising quantum phase transition and a first-order topological phase transition between two topologically distinct Dirac semimetals are observed. Interestingly, at the latter, the Dirac points smear out to form nodal lines in the Brillouin zone, and collective bosonic fluctuations with SO(4) symmetry are strongly enhanced. At strong coupling, these two phase boundaries merge into a first-order transition.

We report on the effect of decoherence on polariton bistability. The polariton hysteresis loop is shown to collapse in a similar way when increasing the temperature or under nonresonant excitation power. The hysteresis upward threshold is pulled to lower excitation power, whereas the downward threshold remains almost constant. This effect is explained by the population of an incoherent reservoir that induces dephasing and repulsive interaction that saturates at large densities. All experimental findings are accurately simulated with the excitonic Bloch equations and indicate that reservoir-induced dephasing can be dominant over the reservoir-induced energy blueshift.