In this work we develop a compact multiorbital tight-binding model for phosphorene that accurately describes states near the main band gap. The model parameters are adjusted using as reference the band structure obtained by a density functional theory calculation with the hybrid HSE06 functional. We use the optimized tight-binding model to study the effects of disorder on the anisotropic transport properties of phosphorene. In particular, we evaluate how the longitudinal resistivity depends on the lattice orientation for two typical disorder models: dilute scatterers with high potential fluctuation amplitudes, mimicking screened charges in the substrate, and dense scatterers with lower amplitudes, simulating weakly bounded adsorbates. We show that the intrinsic anisotropy associated to the band structure of this material, although sensitive to the type and intensity of the disorder, is robust.

The gadolinium pyrochlores ${\mathrm{Gd}}_2{B}_2{\mathrm{O}}_7$ are among the best realizations of antiferromagnetically coupled Heisenberg spins on a pyrochlore lattice. We present a magnetic characterization of ${\mathrm{Gd}}_2{\mathrm{Pt}}_2{\mathrm{O}}_7$, a unique member of this family. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements show that ${\mathrm{Gd}}_2{\mathrm{Pt}}_2{\mathrm{O}}_7$ undergoes an antiferromagnetic ordering transition at $T_N=1.6$ K. This transition is strongly first order, as indicated by the sharpness of the heat capacity anomaly, thermal hysteresis in the magnetic susceptibility, and a nondivergent relaxation rate in $\mu \mathrm{SR}$. The form of the heat capacity below $T_N$ suggests that the ground state is an anisotropic collinear antiferromagnet with an excitation spectrum that is gapped by 0.245(1) meV. The ordering temperature in ${\mathrm{Gd}}_2{\mathrm{Pt}}_2{\mathrm{O}}_7,T_N=1.6$ K, is a substantial 160% increase from other gadolinium pyrochlores, which are all known to order at 1 K or lower. We attribute this enhancement in $T_N$ to the $B$-site cation, platinum. Despite being nonmagnetic, platinum has a filled $5d{t}_{2g}$ orbital and an empty $5d{e}_g$ orbital that can facilitate superexchange. Thus, the magnetic frustration in ${\mathrm{Gd}}_2{\mathrm{Pt}}_2{\mathrm{O}}_7$ is partially “relieved,” thereby promoting magnetic order.

We study the spin-Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in a noncollinear insulating magnet-Pt heterostructure. We show that SMR can be used as an all-electric probe of complex spin states exhibited by the chiral magnet, ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$, under an applied magnetic field. The slope of the magnetic field dependence of the SMR signal changes sign at the transition between the helical and conical spiral states and shows another discontinuity when the conical spiral turns into a collinear ferromagnetic state. We demonstrate that the amplitude of the SMR signal is controlled by the cone angle $\theta$,and that it changes sign at $\theta \sim {55}^{\circ }$. The angular dependence of the SMR in the multidomain helical spiral state is markedly different from the simple sinusoidal dependence observed in the monodomain conical spiral and ferromagnetic states. This complex behavior is explained within the framework of the SMR theory initially developed for collinear magnets. The SSE displays unconventional behavior where not only the magnitude but also the phase of the angular dependence of the SSE varies with the applied magnetic field.

We study the topological stability of nodes in nonsymmorphic superconductors (SCs). In particular, we demonstrate that line nodes in nonsymmorphic odd-parity SCs are protected by the interplay between topology and nonsymmorphic symmetry. As an example, it is shown that the $E_{2u}$-superconducting state of ${\mathrm{UPt}}_3$ hosts the topologically stable line node at the Brillouin zone face. Our theory indicates that the existence of spin-orbit coupling is essential for protecting such a line node, complementing Norman's group theory argument. Developing the topological arguments, we also propose a generalization to point nodes and to other symmetry cases beyond the group theory arguments.

We find a series of topological phase transitions in a half-metal/superconductor heterostructure, by tuning the direction of the magnetization of the half-metal film. These include transitions between a topological superconducting phase with a bulk gap and another phase without a bulk gap but with a ubiquitous local gap. At the same time, the edge states change from counterpropagating Majorana edge modes to unidirectional Majorana edge modes. In addition, we find transitions between the second phase and a nodal phase which turns out to be a two-dimensional Weyl superconductor with Fermi line edge states. We identify the topological invariants relevant to each phase and the symmetry that protects the Weyl superconductivity phase.

We investigate a paradigmatic model for quantum transport with both nearest-neighbor and infinite-range hopping coupling (independent of the position). Due to long-range homogeneous hopping, a gap between the ground state and the excited states can be induced, which is mathematically equivalent to the superconducting gap. In the gapped regime, the dynamics within the excited-state subspace is shielded from long-range hopping, namely it occurs as if long-range hopping would be absent. This is a cooperative phenomenon since shielding is effective over a time scale that diverges with the system size. We named this effect cooperative shielding. We also discuss the consequences of our findings on Anderson localization. Long-range hopping is usually thought to destroy localization due to the fact that it induces an infinite number of resonances. Contrary to this common lore we show that the excited states display strong localized features when shielding is effective even in the regime of strong long-range coupling. A brief discussion on the extension of our results to generic power-law decaying long-range hopping is also given. Our preliminary results confirm that the effects found for the infinite-range case are generic.

We study the level structure of excitations at the “deconfined” critical point separating antiferromagnetic and valence-bond-solid phases in two-dimensional quantum spin systems using the $J\text{−}Q$ model as an example. Energy gaps in different spin $\left(S\right)$ and momentum $\left(\mathbf{k}\right)$ sectors are extracted from imaginary-time correlation functions obtained in quantum Monte Carlo simulations. We find strong quantitative evidence for deconfined linearly dispersing spinons with gapless points at $\mathbf{k}=(0,0),(\pi ,0),(0,\pi )$, and $(\pi ,\pi )$, as inferred from two-spinon excitations ($S=0$ and $S=1$ states) around these points. We also observe a duality between singlet and triplet excitations at the critical point and inside the ordered phases, in support of an enhanced symmetry, possibly SO(5).

Linear Heisenberg antiferromagnets (HAFs) are chains of spin-$S$ sites with isotropic exchange $J$ between neighbors. Open and periodic boundary conditions return the same ground-state energy per site in the thermodynamic limit, but not the same spin $S_G$ when $S\ge 1$. The ground state of open chains of $N$ spins has $S_G=0$ or $S$, respectively, for even or odd $N$. Density-matrix renormalization-group calculations with different algorithms for even and odd $N$ are presented up to $N=500$ for the energy and spin densities $\rho (r,N)$ of edge states in HAFs with $S=1$, 3/2, and 2. The edge states are boundary-induced spin density waves (BI-SDWs) with $\rho (r,N)\propto {(-1)}^{r-1}$ for $r=1,2,...,N$. The SDWs are in phase when $N$ is odd, are out of phase when $N$ is even, and have finite excitation energy $\mathrm{\Gamma }\left(N\right)$ that decreases exponentially with $N$ for integer $S$ and faster than $1/N$ for half integer $S$. The spin densities and excitation energy are quantitatively modeled for integer $S$ chains longer than $5\xi$ spins by two parameters, the correlation length $\xi$ and the SDW amplitude, with $\xi =6.048$ for $S=1$ and 49.0 for $S=2$. The BI-SDWs of $S=3/2$ chains are not localized and are qualitatively different for even and odd $N$. Exchange between the ends for odd $N$ is mediated by a delocalized effective spin in the middle that increases $\left|\mathrm{\Gamma }\right(N\left)\right|$ and weakens the size dependence. The nonlinear sigma model ($\mathrm{NL}\sigma \mathrm{M}$) has been applied to the HAFs, primarily to $S=1$ with even $N$, to discuss spin densities and exchange between localized states at the ends as $\mathrm{\Gamma }\left(N\right)\propto {(-1)}^{N}exp(-N/\xi )$. $S=1$ chains with odd $N$ are fully consistent with the $\mathrm{NL}\sigma \mathrm{M}$; $S=2$ chains have two gaps $\mathrm{\Gamma }\left(N\right)$ with the same $\xi$ as predicted whose ratio is 3.45 rather than 3; the $\mathrm{NL}\sigma \mathrm{M}$ is more approximate for $S=3/2$ chains with even $N$ and is modified for exchange between ends for odd $N$.

We present a method for calculating the energy levels of superconducting circuits that contain highly anharmonic, inductively shunted modes with arbitrarily strong coupling. Our method starts by calculating the normal modes of the linearized circuit and proceeds with numerical diagonalization in this basis. As an example, we analyze the Hamiltonian of a fluxonium qubit inductively coupled to a readout resonator. While elementary, this simple example is nontrivial because it cannot be efficiently treated by the method known as “black-box quantization,” numerical diagonalization in the bare harmonic oscillator basis, or perturbation theory. Calculated spectra are compared to measured spectroscopy data, demonstrating excellent quantitative agreement between theory and experiment.

Excellent fits of the tunneling density of states in disordered superconductors can be often achieved making use of the phenomenological Dynes formula. However, no consistent derivation of this formula has been available so far. The Dynes formula can be interpreted by the simplest causal frequency-dependent gap function $\mathrm{\Delta }\left(\omega \right)$ with a vanishing gap at the Fermi level. Here we show, within the coherent potential approximation, that precisely such a gap function describes superconductors with a Lorentzian distribution of pair-breaking fields and arbitrary potential disorder. We predict spectral and thermodynamic properties of such superconductors.

Wave functions constructed from electron-pair states can accurately model strong electron correlation effects and are promising approaches especially for larger many-body systems. In this article, we analyze the nature and the type of electron correlation effects that can be captured by wave functions restricted to electron-pair states. We focus on the pair-coupled-cluster doubles (pCCD) ansatz also called the antisymmetric product of the 1-reference orbital geminal (AP1roG) method, combined with an orbital optimization protocol presented in Boguslawski et al. [Phys. Rev. B 89, 201106(R) (2014)], whose performance is assessed against electronic structures obtained form density-matrix renormalization-group reference data. Our numerical analysis covers model systems for strong correlation: the one-dimensional Hubbard model with a periodic boundary condition as well as metallic and molecular hydrogen rings. Specifically, the accuracy of pCCD/AP1roG is benchmarked using the single-orbital entropy, the orbital-pair mutual information, as well as the eigenvalue spectrum of the one-orbital and two-orbital reduced density matrices. Our study indicates that contributions from singly occupied states become important in the strong correlation regime which highlights the limitations of the pCCD/AP1roG method. Furthermore, we examine the effect of orbital rotations within the pCCD/AP1roG model on correlations between orbital pairs.

Conventional superconductors disordered by magnetic impurities demonstrate physical properties that are drastically different from their pristine counterparts. In our previous work [D. Persson et al., Phys. Rev. B 92, 245430 (2015)], we explored the spectral and thermodynamic properties of such systems for two extreme cases: completely random and ferromagnetically aligned impurity magnetic moments. Here we consider the transport properties of these systems and show that they have a potential to be used in superconducting spintronic devices. Each magnetic impurity contributes a Yu-Shiba-Rusinov (YSR) bound state to the spectrum, residing at subgap energies. Provided the YSR states form metallic bands, we demonstrate that the tunneling current carried by these states can be highly spin polarized when the impurities are ferromagnetically ordered. The spin polarization can be switched by tuning the bias voltage. Moreover, even when the impurity spins are completely uncorrelated, one can still achieve almost $100\%$ spin polarization of the current, if the tunnel interface is spin active. We compute electric current and noise, varying parameters of the interface between tunneling and fully transparent regimes, and analyze the relative role of single-particle and Andreev reflection processes.

Rocksalt structured chalcogenides, such as PbTe, PbSe, and SnTe, are the top candidates for midtemperature thermoelectric applications, and their $p$-type thermoelectric efficiencies can be enhanced via aligning the valence bands. Here, we provided comprehensive numerical investigations on the effects of band convergence on electronic properties. We found that the extra valance band can indeed significantly enhance the power factor. Nevertheless, the extra valance band can also increase the electronic thermal conductivity, which partially offsets the enhanced power factor for the overall figure of merit. Finally, we predicted that the maximum figure of merit for PbTe, PbSe, and SnTe can reach 2.2, 1.8, and 1.6, respectively, without relying on the reduction in lattice thermal conductivity.

Three-dimensional Dirac semimetals host bulk Dirac fermions characterized by a linear dispersion relation in momentum space along all three dimensions. It has been theoretically predicted that the breaking of $C_4$ rotational crystalline symmetry in the Dirac semimetal $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ may give rise to the massive Dirac fermions. Here we report the phase-coherent transport in $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ nanowires studied by measuring the universal conductance fluctuations (UCFs). It is found that the UCF amplitude is largely suppressed at the Dirac point by sweeping the magnetic field at different gate voltages, which is ascribed to the breaking of the $C_4$ rotational symmetry-induced band-gap opening. The temperature dependence of resistance demonstrates a magnetic-field-induced metal-insulator transition, consisting of the band-gap opening. Moreover, the UCF amplitude is reduced by a factor of $\sim 2\sqrt{2}$ in the presence of a magnetic field, suggesting the phase transition from a Dirac-to-Weyl semimetal by breaking time-reversal symmetry.

We present a type of transmon split-junction qubit which can be tuned by Meissner screening currents flowing in the adjacent superconducting film electrodes. The best detected relaxation time $\left(T_1\right)$ was of the order of $50\mu \mathrm{s}$ and the dephasing time $\left(T_2\right)$ about $40\mu \mathrm{s}$. The achieved period of oscillation with magnetic field was much smaller than in the usual SQUID-based transmon qubits; thus a strong effective field amplification has been realized. This Meissner qubit allows a strong mixing of the current flowing in the qubit junctions and the currents generated by the Abrikosov vortices. We present a quantitative analysis of the radiation-free relaxation in qubits coupled to the Abrikosov vortices. The observation of coherent quantum oscillations provides strong evidence that the position of the vortex as well as its velocity do not have to accept exact values but can be smeared in the quantum mechanical sense. The eventual relaxation of such states contributes to an increased relaxation rate of the qubit coupled to vortices. Such relaxation is described using basic notions of the Caldeira-Leggett quantum dissipation theory.

Several promising applications of amorphous titanium dioxide, $a{\mathrm{TiO}}_2$, have appeared recently, but the correlation between electronic properties and atomic short-range structural order is poorly understood. Herein we show that structural disorder yields local undercoordinated ${\mathrm{TiO}}_x$ units which influence electronic hybridization of $\text{Ti-}\left[4p\right]$ and $\text{Ti-}\left[3d\right]$ orbitals with a low crystal-field splitting $[E\left(e_g\right)-E\left(t_{2g}\right)=2.4\pm 0.3\mathrm{eV}]$. The short-range order and electronic properties of $a{\mathrm{TiO}}_2$ thin-film oxides are described through an integrated approach based on x-ray-absorption experiments and ab initio computational simulations where the energy splitting of the electronic levels in the $\text{Ti-}[4p-3d]$ manifold are analyzed. Structural disorder provides enough $p-d$ orbital mixing for the hybridized electronic transitions from the $\text{Ti-}\left[1s\right]$ core level into the $[\mathrm{Ti}-t_{2g}]$ and $[\mathrm{Ti}-e_g]$ bands [$1s\to 4p-3d$ excitations], to be allowed. This yields an intense pre-edge structure in the Ti $K$-edge x-ray-absorption near-edge structure spectrum of $a{\mathrm{TiO}}_2$, which is consistent with the projected density of states on the photoabsorbing Ti atoms.

Affleck, Kennedy, Lieb, and Taski (AKLT) constructed an exemplary spin-3/2 valence-bond solid (VBS) state on the hexagonal lattice, which is the ground state of an isotropic quantum antiferromagnet and possesses no spontaneous magnetization but finite correlation length. This is distinct from the Néel ordered state of the spin-3/2 Heisenberg model on the same lattice. Niggemann, Klümper, and Zittartz then generalized the AKLT Hamiltonian to one family invariant under spin rotation about the $z$ axis. The ground states of this family can be parameterized by a single parameter that deforms the AKLT state, and this system exhibits a quantum phase transition between the VBS and Néel phases, as the parameter increases from the AKLT point to large anisotropy. We investigate the opposite regime when the parameter decreases from the AKLT point and find that there appears to be a Berezinskii-Kosterlitz-Thouless-like transition from the VBS phase to an XY phase. Such a transition also occurs in the deformation of other types of AKLT states with triplet-bond constructions on the same lattice. However, we do not find such an XY-like phase in the deformed AKLT models on other trivalent lattices, such as square-octagon, cross, and star lattices. On the star lattice, the deformed family of AKLT states remain in the same phase as the isotropic AKLT state throughout the whole region of the parameter. However, for two triplet-bond generalizations, the triplet VBS phase is sandwiched between two ferromagnetic phases (for large and small deformation parameters, respectively), which are characterized by spontaneous magnetizations along different axes. Along the way, we also discuss how various deformed AKLT states can be used for the purpose of universal quantum computation.