We study the role of impurities in a two-band superconductor, and elucidate the nature of the recently predicted transition from the $s_{\pm }$ state to the $s_{++}$ state induced by interband impurity scattering. Using a Ginzburg-Landau theory, derived from microscopic equations, we demonstrate that close to $T_c$ this transition is necessarily a direct one, but deeper in the superconducting state an intermediate complex state appears. This state has a distinct order parameter, which breaks the time-reversal symmetry, and is separated from the $s_{\pm }$ and $s_{++}$ states by continuous phase transitions. Based on our results, we suggest a phase diagram for systems with weak repulsive interband pairing, and discuss its relevance to iron-based superconductors.

We study crystal nucleation in metallic niobium supercooled liquid by ab initio molecular dynamics simulations. Using various structural analysis methods including bond orientational order analysis, a scheme of crystallization process is revealed. Extended structured regions of high bond orientational order are spontaneously formed in a supercooled state by thermal fluctuations. This order mainly has bcc-type local symmetry, but a small portion has hcp-type local symmetry. Since these regions act as precursors for crystallization, homogeneous nucleation takes place quite heterogeneously. This preexisting order further selects a main crystal polymorph (bcc) to be formed upon future crystallization. This crystallization behavior is consistent with that recently reported for hard-sphere (colloidal) as well as soft-sphere (Gaussian core model) systems, suggesting the generality.

We study the time evolution of entanglement entropy and entanglement spectrum in a finite-size system which crosses a quantum phase transition at different speeds. We focus on the transverse-field Ising model with a time-dependent magnetic field, which is linearly tuned on a time scale $\tau$. The time evolution of the entanglement entropy displays different regimes depending on the value of $\tau$, showing also oscillations which depend on the instantaneous energy spectrum. The entanglement spectrum is characterized by a rich dynamics where multiple crossings take place with a gap-dependent frequency. Moreover, we investigate the Kibble-Zurek scaling of the entanglement entropy and Schmidt gap.

We present an exact solution for a class of one-dimensional compass models which stand for interacting orbital degrees of freedom in a Mott insulator. By employing the Jordan-Wigner transformation we map these models on noninteracting fermions and discuss how spin correlations, high degeneracy of the ground state, and $Z_2$ symmetry in the quantum compass model are visible in the fermionic language. Considering a zigzag chain of ions with singly occupied $e_g$ orbitals ($e_g$ orbital model) we demonstrate that the orbital excitations change qualitatively with increasing transverse field, and that the excitation gap closes at the quantum phase transition to a polarized state. This phase transition disappears in the quantum compass model with maximally frustrated orbital interactions which resembles the Kitaev model. Here we find that the finite transverse field destabilizes the orbital-liquid ground state with macroscopic degeneracy, and leads to peculiar behavior of the specific heat and orbital susceptibility at finite temperature. We show that the entropy and the cooling rate at finite temperature exhibit quite different behavior near the critical point for these two models.

The evolution of the electronic structure and magnetic properties with Co substitution for Fe in the solid solution Fe${}_{1-x}$Co${}_x$Ga${}_3$ was studied by means of ab initio band-structure calculations and nuclear spin-lattice relaxation $1/T_1$ of the ${}^{69,71}$Ga nuclei. The ${}^{69,71}(1/T_1)$ was studied as a function of temperature in a wide temperature range of 2–300 K for the concentrations $x=0.0$, 0.5, and 1.0. In the parent semiconducting compound FeGa${}_3$, the temperature dependence of the ${}^{69}(1/T_1)$ exhibits a huge maximum at about $T\sim 6$ K indicating the existence of in-gap states. The opposite binary compound, CoGa${}_3$, demonstrates a metallic Korringa behavior with $1/T_1\propto T$. In Fe${}_{0.5}$Co${}_{0.5}$Ga${}_3$, the relaxation is strongly enhanced due to spin fluctuations and follows $1/T_1\propto T^{1/2}$, which is a unique feature of weakly and nearly antiferromagnetic metals. This itinerant antiferromagnetic behavior contrasts with ab initio band-structure calculations, where a ferromagnetic state with an ordered moment of 0.5 ${\mu }_B$/f.u. is predicted. The results are discussed in terms of the interplay between the localized and itinerant magnetism including in-gap states and spin fluctuations.

Magnetic, structural, and magnetocaloric properties have been investigated for a solid-solution system, ${\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{V}}_2{\mathrm{O}}_4$ ($0.0\le x\le 1.0$) with a spinel structure. As orbital-active ${\mathrm{Fe}}^{2+}$ ions are partially substituted with orbital-inactive ${\mathrm{Mn}}^{2+}$ ions, various interactions, such as the Jahn–Teller interaction, spin-orbit coupling, and the exchange interaction, compete with each other, giving rise to a rich magnetic and structural phase diagram. The magnetocaloric effect exhibits two peaks as a function of temperature for $x\le 0.9$, associated with a higher-temperature ferrimagnetic transition, and with a lower-temperature concomitant spin-canting and orbital-ordering (mostly lattice-structural) transition of the V site. The large magnetocaloric effect as observed in $\mathrm{Mn}{\mathrm{V}}_2{\mathrm{O}}_4$ can thus be attributed to the sum of the entropy changes upon the merged phase transition at $T_{\mathrm{N}1}\sim T_{\mathrm{N}2}$.

Quasiparticle interference (QPI), by means of scanning tunneling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and multiorbital tight-binding calculations, is used to investigate the band structure and superconducting order parameter of LiFeAs. Using this combination of techniques we identify intraband and interband scattering vectors between the hole ($h$) and electron ($e$) bands in the QPI maps. Discrepancies in the band dispersions inferred from previous ARPES and STM/STS are reconciled by recognizing a difference in the $k_z$ sensitivity for the two probes. The observation of both $h$-$h$ and $e$-$h$ scattering is exploited using phase-sensitive scattering selection rules for Bogoliubov quasiparticles. From this we demonstrate an $s_{\pm }$ gap structure, where a sign change occurs in the superconducting order parameter between the $e$ and $h$ bands.

An array of quantum wires is a natural starting point in realizing two-dimensional topological phases. We study a system of weakly coupled quantum wires with Rashba spin-orbit coupling, proximity coupled to a conventional $s$-wave superconductor. A variety of topological phases are found in this model. These phases are characterized by “strong” and “weak” topological invariants that capture the appearance of midgap Majorana modes (either chiral or nonchiral) on edges along and perpendicular to the wires. In particular, a phase with a single chiral Majorana edge mode (analogous to a $p+ip$ superconductor) can be realized. At special values of the magnetic field and chemical potential, this edge mode is almost completely localized at the outmost wires. In addition, a phase with two copropagating chiral edge modes is observed. We also consider ways to distinguish experimentally between the different phases in tunneling experiments.

We demonstrate how the generalized Pauli exclusion principle emerges for quasiparticle excitations in 2D topological phases. As an example, we examine the Levin-Wen model with the Fibonacci data (specified in the text), and construct the number operator for fluxons living on plaquettes. By numerically counting the many-body states with fluxon number fixed, the matrix of exclusion statistics parameters is identified and is shown to depend on the spatial topology (sphere or torus) of the system. Our work reveals the structure of the (many-body) Hilbert space and some general features of thermodynamics for quasiparticle excitations in topological matter.

Using linear response theory with the dynamical mean-field approximation we investigate the particle-hole instabilities of the two-band Hubbard model in the vicinity of the spin-state transition. Besides the previously reported high-spin–low-spin order we find an instability towards triplet excitonic condensate. We discuss the strong and weak-coupling limits of the model, in particular, a connection to the spinful hard-core bosons with a nearest-neighbor interaction. Possible realization in perovskite cobaltites is briefly discussed.

NpCoGe, the neptunium analog of the ferromagnetic superconductor UCoGe, has been investigated by dc magnetization, ac susceptibility, specific heat, electrical resistivity, Hall effect, Np Mössbauer spectroscopy, and local spin-density approximation (LSDA) calculations. NpCoGe exhibits an antiferromagnetic ground state with a Néel temperature $T_N\approx 13$ K and an average ordered magnetic moment $\langle {\mu }_{\mathrm{Np}}\rangle =0.80{\mu }_B$. The magnetic phase diagram has been determined and shows that the antiferromagnetic structure is destroyed by the application of a magnetic field (≈3 T). The value of the isomer shift suggests a Np${}^{3+}$ charge state (configuration $5f^4$). A high Sommerfeld coefficient value for NpCoGe (170 mJ mol${}^{-1}$ K${}^{-2}$) is inferred from specific heat. LSDA calculations indicate strong magnetic anisotropy and easy magnetization along the $c$ axis. Mössbauer data and calculated exchange interactions support the possible occurrence of an elliptical spin-spiral structure in NpCoGe. The comparison with NpRhGe and uranium analogs suggests the leading role of 5f-d hybridization, the rather delocalized character of 5f electrons in NpCoGe, and the possible proximity of NpRuGe or NpFeGe to a magnetic quantum critical point.

Variational wave functions that introduce zeros (vortices) to screen repulsive interactions are typically difficult to verify in unbiased microscopic calculations. An approach is constructed to insert vortices into ansatz wave functions using a matrix-product representation. This approach opens the door to validation of a broad class of Jastrow-based wave functions. The formalism is applied to a model motivated by experiments on ultracold atomic gases in the presence of synthetic spin-orbit coupling. Validated wave functions show that vortices in atomic Fermi gases with flat Rashba spin-orbit bands cluster near the system center and should therefore be directly visible in time-of-flight imaging.

We show that conservation laws in quantum mechanics naturally lead to metric spaces for the set of related physical quantities. All such metric spaces have an “onion-shell” geometry. We demonstrate the power of this approach by considering many-body systems immersed in a magnetic field, with a finite ground state current. In the associated metric spaces we find regions of allowed and forbidden distances, a “band structure” in metric space directly arising from the conservation of the $z$ component of the angular momentum.

We theoretically analyze the d.c. Josephson current between two superconductors connected by a vibrating weak link. Assuming the weak link to be a “quantum dot”, we predict that the critical current is thermally enhanced at low temperatures if the electron-vibron interaction is strong and has an anomalous temperature dependence in a large temperature interval. We estimate that this unusual behavior, which occurs because the current through the weak link is carried by “polaronic” Andreev states, is measurable in experiments on, e.g., carbon-nanotube-based Josephson junctions.

We present two methods for computing two-time correlation functions or Green's functions from real-time bold-line continuous-time quantum Monte Carlo. One method is a formally exact generalized auxiliary lead formalism by which spectral properties may be obtained from single-time observables. The other involves the evaluation of diagrams contributing to two-time observables directly on the Keldysh contour. Additionally, we provide a detailed description of the bold-line Monte Carlo method. Our methods are general and numerically exact, and able to reliably resolve high-energy features such as band edges. We compare the spectral functions obtained from real-time methods to analytically continued spectral functions obtained from imaginary-time Monte Carlo, thus probing the limits of analytic continuation.

The development of silicon-based nanoscale technology for the realization of single electron, single spin quantum devices demands deep donor-based systems to achieve a major breakthrough in the field: high-temperature operation. Here, we suggest that, despite some preparation difficulties, substitutional nitrogen in silicon (${\mathrm{N}}_{\text{Si}}$) represents an interesting candidate for this purpose, being observable by electron paramagnetic resonance (EPR) at room temperature. We report a study of the nature and dynamics of substitutional nitrogen in silicon, the so-called SL5 paramagnetic center, by X-band continuous-wave EPR, complemented by pulsed EPR. Both natural and ${}^{28}\mathrm{Si}$ isotopically enriched nitrogen-doped silicon samples have been used, the latter providing an improvement in the accuracy of the spin Hamiltonian parameters.

The Mn dopant distribution in ultrathin (20 nm) highly doped (nominal $x$ = 0.20) ${Ga}_{1-x}$Mn${}_x$As epitaxial films with critical temperatures close to 175 K and magnetization of 100 emu/cm${}^3$ is analyzed by Rutherford backscattering spectrometry (RBS) in a random and channeling configuration. We could quantify the total concentration and the respective fraction of substitutional, interstitial, and random site Mn ions in as-grown and annealed samples. The measured total Mn concentration is $x$ = 0.23. In the as-grown state 30% of the Mn dopant is located on interstitial sites. Thermal annealings at 180 °C for several hours monotonically reduce the interstitial Mn fraction to 11%. Simultaneously the fraction of randomly located Mn is increased by the same amount. The substitutional Mn concentration is stable under these annealing conditions. The effective Mn concentration could be increased to $x$ = 0.13. However, the critical temperature does not increase proportionally with the magnetization. A comparison of the magnetization values demonstrates that the interstitial Mn ions are already incorporated during the growth in the form of Mn${}_{\mathrm{Ga}}$-Mn${}_i$ clusters.

We investigate the wave functions, spectrum, and $g$-factor anisotropy of low-energy electrons confined to self-assembled, pyramidal InAs quantum dots (QDs) subject to external magnetic and electric fields. We present the construction of trial wave functions for a pyramidal geometry with hard-wall confinement. We explicitly find the ground and first excited states and show the associated probability distributions and energies. Subsequently, we use these wave functions and 8-band $\mathit{k}·\mathbf{p}$ theory to derive a Hamiltonian describing the QD states close to the valence band edge. Using a perturbative approach, we find an effective conduction band Hamiltonian describing low-energy electronic states in the QD. From this, we further extract the magnetic field dependent eigenenergies and associated $g$ factors. We examine the $g$ factors regarding anisotropy and behavior under small electric fields. In particular, we find strong anisotropies, with the specific shape depending strongly on the considered QD level. Our results are in good agreement with recent measurements [Takahashi et al., Phys. Rev. B 87, 161302 (2013)] and support the possibility to control a spin qubit by means of $g$-tensor modulation.

Employing density functional calculations we predict that SmS undergoes a topological phase transition from the trivial Kondo insulator (KI) black phase to a topological metallic gold phase under hydrostatic pressure. The underlying mechanism is the pressure-induced change of the 4$f$ level from below to above the bottom of the 5$d$ conduction band, leading to a $d$-$f$ band inversion, a parity sign reversal, and the concomitant change of the topological invariant ${\nu }_0$. This provides a material realization of the topological classification of KIs proposed by Dzero et al.

We study the finite-temperature antiferromagnetic phase of the ionic Hubbard model in the strongly interacting limit using quantum Monte Carlo based dynamical mean-field theory. We find that the ionic potential plays a dual role in determining the antiferromagnetic order. A small ionic potential (compared to Hubbard repulsion) increases the superexchange coupling in the projected sector of the model, leading to an increase in the Neel temperature of the system. A large ionic potential leads to resonance between projected antiferromagnetically ordered configurations and density ordered configurations with double occupancies, thereby killing antiferromagnetism in the system. This novel way of degrading antiferromagnetism leads to spin polarization of the low-energy single-particle density of states. The dynamic response of the system thus mimics ferromagnetic behavior, although the system is still an antiferromagnet in terms of the static spin order.