The 1111-type iron arsenide hydride CaFe${}_{1-x}$Co${}_x$AsH was synthesized by high-pressure solid-state reaction, and its electronic structure and superconducting properties were investigated. Bulk superconductivity was observed at $x$ = 0.09–0.26. A maximum superconducting critical temperature ($T_{\mathrm{c}}^{max}$) of 23 K was observed at $x$ = 0.09. These values are in agreement with those of CaFe${}_{1-x}$Co${}_x$AsF. The calculated Fermi surface of CaFeAsH has a small three-dimensional (3D) hole pocket around the Γ point. This is a result of weak covalent bonding between the As 4$p$ and H 1$s$ orbitals. No such covalency exists in CaFeAsF, because the energy level of the F 2$p$ orbital is sufficiently deep to inhibit overlap with the As 4$p$ orbital. The similar superconductivities of CaFe${}_{1-x}$Co${}_x$AsH and CaFe${}_{1-x}$Co${}_x$AsF are explained with the nesting scenario. The small 3D hole pocket of CaFe${}_{1-x}$Co${}_x$AsH does not significantly contribute to electron excitation. These findings encourage exploration of hydrogen-containing 1111-type iron-based materials with lower anisotropies and higher $T_c$ applicable to superconducting wires and tapes.

Intense ultrashort extreme ultraviolet (XUV) pulses can be used to create warm dense matter in the laboratory, which then develops to a plasma state. So far, however, it is unknown, whether this transition occurs via heat transfer from hot electrons to cold atoms or nonthermally due to a lattice instability. Here we computed the response of the phonon spectra of copper and silver to the presence of XUV-excited core holes and core holes together with very hot electrons. We found that the average interatomic bonds become stronger in the warm dense state. We discuss why these findings support the above-mentioned heat transfer scenario.

We have measured the nuclear spin-lattice relaxation of ${}^{59}$Co in the geometrically frustrated Ising-type spin-chain system Ca${}_3$Co${}_2$O${}_6$ as a function of temperature and applied magnetic field. In the nearly-saturated ferrimagnetic and ferromagnetic regimes considered here, the field-dependent energy scales governing the spin-lattice relaxation rates reflect the energy cost of a spin flip in the Co ions adjacent to the probed nuclei. This results in a thermally activated form for the nuclear relaxation rates, which can be exploited as a local probe of the exchange interactions. In particular, the measurements enable the values of intra- and interchain exchange constants $J_1$, $J_2+J_3$ to be directly extracted from the data. By using a quasi-one-dimensional model, we then determine the value of $J_2$ consistent with the spin-density-wave order observed in zero field.

We study the dynamics of head-to-head domain walls separating in-plane domains in a disordered ferromagnetic thin film. The competition between the domain-wall surface tension and dipolar interactions induces a crossover between a rough domain-wall phase at short length scales and a large-scale phase where the walls display a zigzag morphology. The two phases are characterized by different critical exponents for Barkhausen avalanche dynamics that are in quantitative agreement with experimental measurements on MnAs thin films.

The analysis of nonclassical rotational response of superfluids and superconductors was performed by Onsager [Onsager, Nuovo Cimento, Suppl. 6, 279 (1949)] and London [Superfluids (Wiley, New York, 1950)] and crucially advanced by Feynman [Prog. Low Temp. Phys. 1, 17 (1955)]. It was established that, in the thermodynamic limit, neutral superfluids rotate by forming—without any threshold—a vortex lattice. In contrast, the rotation of superconductors at angular frequency $\mathit{\Omega }$—supported by uniform magnetic field ${\mathbf{B}}_L\propto \mathit{\Omega }$ due to surface currents—is of the rigid-body type (London law). Here we show that, neglecting the centrifugal effects, the behavior of a rotating superconductor is identical to that of a superconductor placed in a uniform fictitious external magnetic field $\tilde{\mathbf{H}}=-{\mathbf{B}}_L$. In particular, the isomorphism immediately implies the existence of two critical rotational frequencies in type-2 superconductors.

Recently, we have proposed the half-Josephson laser (HJL): a device that combines lasing with superconducting leads, providing a locking between the optical phase and the superconducting phase difference between the leads. In this work, we propose and investigate two setups derived from a superconducting quantum interference device (SQUID), where two conventional Josephson junctions are replaced by two HJLs. In the first setup, the HJLs share the same resonant mode, while in the second setup two separate resonant modes of the two lasers are coupled optically. We dub the setup “light-superconducting interference device” (LSID). In both setups, we find the operating regimes similar to those of a single HJL. Importantly, the steady lasing field is significantly affected by the magnetic flux penetrating the SQUID loop, with respect to both amplitude and phase. This provides opportunities to tune or even quench the lasing by varying a small magnetic field. For the second setup, we find a parameter range where the evolution equation for the laser fields supports periodic cycles. The fields are thus modulated with the frequency of the cycle resulting in an emission spectrum consisting of a set of discrete modes. From this spectrum, two modes dominate in the limit of strong optical coupling. Therefore, the LSID can be also used to generate such modulated light.

We apply mean-field theory and the Hirsch-Fye quantum Monte Carlo method to study the spin-spin interaction in the bulk of three-dimensional topological insulators. We find that the spin-spin interaction has three different components: longitudinal, transverse, and transverse Dzyaloshinskii-Moriya-like terms. When the Fermi energy is located in the bulk gap of a topological insulator, the spin-spin interaction decays exponentially due to the Bloembergen-Rowland interaction. The longitudinal correlation is antiferromagnetic and the transverse correlations are ferromagnetic. When the chemical potential is in the conduction or valence band, the spin-spin interaction follows a power-law decay, and the isotropic ferromagnetic interaction dominates in the short separation limit.

We argue that various kinds of topological insulators (TIs) can be insightfully characterized by an inspection of the charge centers of the hybrid Wannier functions, defined as the orbitals obtained by carrying out a Wannier transform on the Bloch functions in one dimension while leaving them Bloch-like in the other two. From this procedure, one can obtain the Wannier charge centers (WCCs) and plot them in the two-dimensional projected Brillouin zone. We show that these WCC sheets contain the same kind of topological information as is carried in the surface energy bands, with the crucial advantage that the topological properties of the bulk can be deduced from bulk calculations alone. The distinct topological behaviors of these WCC sheets in trivial, Chern, weak, strong, and crystalline TIs are first illustrated by calculating them for simple tight-binding models. We then present the results of first-principles calculations of the WCC sheets in the trivial insulator ${\mathrm{Sb}}_2$Se${}_3$, the weak TI KHgSb, and the strong TI ${\mathrm{Bi}}_2$Se${}_3$, confirming the ability of this approach to distinguish between different topological behaviors in an advantageous way.

Strained transition layers, which are common for heteroepitaxial growth of functional semiconductors on foreign substrates, include high defect densities that impair heat conduction. Here, we measure the thermal resistances of AlN transition layers for GaN on Si and SiC substrates in the temperature range $300<T<550$ K using time-domain thermoreflectance. We propose a model for the effective resistance of such transition films, which accounts for the coupled effects of phonon scattering on defects and the two interfaces. The data are consistent with this model using point defects at concentrations near 10${}^{20}$ cm${}^{-3}$ and transmission coefficients based on the diffuse mismatch model. The data can be also described using lower transmission coefficients and eliminating the defects in the AlN. The data and modeling support the hypothesis that point defect scattering in the AlN film dominates the resistance, but may also be consistent with a high presence of near-interfacial defects in the bounding films.

We compute the electromagnetic fluctuations due to evanescent-wave Johnson noise in the vicinity of a thin conducting film, such as a metallic gate or a two-dimensional electron gas. This noise can decohere a nearby qubit, and it is also responsible for heat transfer and Casimir forces. We have improved on previous calculations of decoherence rates by including the nonlocal dielectric response of the film, which is an important correction at short distances. Remarkably, the fluctuations responsible for decoherence of charge qubits from a thin film are greatly enhanced over those arising from a conducting half space. The decoherence times can be reduced by over an order of magnitude by decreasing the film thickness. This appears to be due to the leakage into the vacuum of modes that are well localized in the perpendicular direction. There is no corresponding effect for spin qubits (magnetic field fluctuations). We also show that a nonlocal dielectric function naturally removes the divergence in the Casimir force at vanishing separation between two metallic sheets or half spaces. In the separation regime where local and nonlocal treatments are noticeably distinct, the Casimir attraction between two thin sheets and two half spaces is practically indistinguishable for any physical film thickness.

The nature of the electronic ground state of Ba${}_2$IrO${}_4$ has been addressed using soft x-ray absorption and inelastic scattering techniques in the vicinity of the oxygen $K$ edge. From the polarization and angular dependence of x-ray absorption spectroscopy (XAS) we deduce an approximately equal superposition of $xy$, $yz$, and $zx$ Ir${}^{4+}$ $5d$ orbitals. By combining the measured orbital occupancies, with the value of the spin-orbit coupling provided by resonant inelastic x-ray scattering (RIXS), we estimate the crystal field splitting associated with the tetragonal distortion of the IrO${}_6$ octahedra to be small, $\Delta \approx 50\left(50\right)$ meV. We thus conclude definitively that Ba${}_2$IrO${}_4$ is a close realization of a spin-orbit Mott insulator with a $j_{\mathrm{eff}}=\frac{1}{2}$ ground state, thereby overcoming ambiguities in this assignment associated with the interpretation of x-ray resonant scattering experiments.

We consider the time evolution after quantum quenches in the spin-$\frac{1}{2}$ Heisenberg XXZ quantum spin chain with Ising-type anisotropy. The time evolution of short-distance spin-spin correlation functions is studied by numerical tensor network techniques for a variety of initial states, including Néel and Majumdar-Ghosh states and the ground state of the XXZ chain at large values of the anisotropy. The various correlators appear to approach stationary values, which are found to be in good agreement with the results of exact calculations of stationary expectation values in appropriate generalized Gibbs ensembles. In particular, our analysis shows how symmetries of the post-quench Hamiltonian that are broken by particular initial states are restored at late times.

In order to account explicitly for the existence of long-periodic layered structures and the strong structural relaxations in the most common binary and ternary alloys of the Bi-Sb-Te-Se system, we have developed a one-dimensional cluster expansion (CE) based on first-principles electronic structure calculations, which accounts for the Bi and Sb bilayer formation. Excellent interlayer distances are obtained with a van der Waals density functional. It is shown that a CE solely based on pair interactions is sufficient to provide an accurate description of the ground-state energies of Bi-Sb-Te-Se binary and ternary systems without making the data set of ab initio calculated structures unreasonably large. For the binary alloys $A_{1-x}{Q}_x$ ($A=\text{Sb}$, Bi; $Q=\text{Te}$, Se), a ternary CE yields an almost continuous series of (meta)stable structures consisting of consecutive $A$ bilayers next to consecutive $A_2{Q}_3$ for $0<x<0.6$. For $x>0.6$, the binary alloy segregates into pure $Q$ and $A_2{Q}_3$. The Bi-Sb system is described by a quaternary CE and is found to be an ideal solid solution stabilized by entropic effects at $T\ne 0$ K but with an ordered structure of alternating Bi and Sb layers for $x=0.5$ at $T=0$ K. A quintuple CE is used for the ternary Bi-Sb-Te system, where stable ternary layered compounds with an arbitrary stacking of ${\text{Sb}}_2$${\text{Te}}_3$, ${\text{Bi}}_2$${\text{Te}}_3$, and Te-Bi-Te-Sb-Te quintuple units are found, optionally separated by mixed Bi/Sb bilayers. Electronic properties of the stable compounds were studied taking spin-orbit coupling into account.

The magnetism of the $M_{n+1}A{X}_n$ phase, ${\text{Cr}}_2$GeC, and its Mn-doped system, (${\text{Cr}}_{1-x}$${\text{Mn}}_x$)${}_2$GeC ($x\le 0.25$), synthesized via a solid state reaction, was investigated systematically. ${\text{Cr}}_2$GeC is in a spin-unpolarized state, but the ferromagnetic band polarization is induced immediately by the Mn doping. The Curie temperature, $T_{\mathrm{C}}$, and the spontaneous moment, $p_{\mathrm{s}}$, increase almost proportionally to the Mn concentration, strongly suggesting that ${\text{Cr}}_2$GeC is located in the vicinity of a ferromagnetic quantum critical point. The strong concentration dependence of $p_{\mathrm{eff}}/p_{\mathrm{s}}$, where $p_{\mathrm{eff}}$ is the effective moment in the paramagnetic state, indicates that the ferromagnetism appearing in the Mn-doped ${\text{Cr}}_2$GeC can be classified as a typical itinerant-electron ferromagnetism in a wide range of the degree of electron localization.

Magnetoelectric coupling at the interface between $\text{PbTi}\left(\text{Zr}\right){\text{O}}_3$ (001) (PZT) and a Co film as well as its tuning by Zr substitutions is studied here from first principles. Chemically disordered 25% Zr-doped compositions were modeled within the supercell approach. Our calculations suggest that ferroelectric ionic displacements in PZT can be locally enhanced, especially when the interfacial Ti cations are replaced with Zr. Regarding Co films their structure tends to become hexagonal for thicker films, while rather significant changes in Co magnetization are anticipated in response to the electric polarization reversal in PZT.

Magnetic-field-induced spin flop transition and magnetoelectric effect in noncentrosymmetric ${\text{Ca}}_2$${\text{Fe}}_{2-x}$${\text{Al}}_x$${\text{O}}_5$ (0.5 $\le$ $x$ $\le$ 1) single crystals have been investigated. Temperature dependence of magnetic susceptibility shows that the compounds have antiferromagnetic transition temperatures $T_N$ above room temperature. The spin easy axis varies from the $c$ axis to the $a$ axis with increasing Al content $x$. When a magnetic field is applied along the spin easy axis, a spin flop transition takes place, accompanied by anomalies in the electric polarization and dielectric constant. The anomalies can be ascribed to a noncollinear spin arrangement in the domain walls between two magnetic phases and/or the spin direction dependent modulation of the metal-ligand hybridization. Such a polarization change with a spin flop transition in polar antiferromagnets may provide a route to realize large magnetoelectric coupling at high temperatures.

Using a microwave power reflection technique, we have experimentally investigated the dynamics of vortices in a patterned superconductor consisting in a thin film of lead with a periodic array of microholes (antidots). Commensurability effects at integer and fractional matching field values have been observed as peaks in the magnetic field dependence of the microwave backward reflection coefficient for different values of temperature, frequency, and power. These peaks appear as a result of the contribution of the vortex dynamics to the reflected signal at the matching fields and their observation depends on the values of the microwave power supplied to the sample.

We report comprehensive (magneto)transport studies of the two-phase state in ${\left(\text{TMTSF}\right)}_2$${\text{ClO}}_4$, where superconducting (SC) phase coexists with spin-density wave insulator (SDW). By tuning the degree of ${\text{ClO}}_4$ anion ordering in controlled manner we smoothly suppress the SDW state and study resulting evolution of the SC phase spatial texture. We find that as SDW is suppressed, SC regions initially appear inside the SDW insulator in a form of filaments extended in the interlayer direction and further merge into the two-dimensional sheets across the most conducting axis of the crystal. We demonstrate that almost all our results can be explained within the soliton phase model, though with several assumptions, they can also be related with the creation of nonuniform deformations. We believe that the anisotropy is intrinsic to SC/SDW coexistence in various quasi-one-dimensional superconductors.

From first-principles calculations we show that there is an intimate relationship between the Se height and the spin configuration of the underlying Fe square in $M_x$Fe${}_{2-y}$Se${}_2$, where $M$ denotes an alkali metal. A displacement of the Se atom by an amount as small as 0.2 Å is enough to change the amount of charge in the Fe plane by as much as 0.8 electrons per Fe. The height of the Se atom above the Fe square increases as the number of ferromagnetic Fe-Fe bonds increases, yielding an expansion of 2 Å in the $c$ axis for the fully ferromagnetic spin configuration, which suggests the presence of a giant magnetoelastic coupling in this system. Interestingly, the strong coupling between the Se height and the spin configuration of the underlying Fe square is found to be rather local, such that both the energetics and the structural parameters of the $\sqrt{5}\times \sqrt{5}$ phase can be understood in terms of three different magnetic configurations of the ideal 122 structure. We show that the formation of Fe vacancies is energetically favorable and that their presence changes the ground state from double-stripe to block-antiferromagnetic ordering. The formation of K vacancies is found to be energetically unfavorable.

Two interacting particles (TIP) in a disordered chain propagate beyond the single-particle localization length ${\xi }_1$ up to a scale ${\xi }_2>{\xi }_1$. An initially strongly localized TIP state expands almost ballistically up to ${\xi }_1$. The expansion of the TIP wave function beyond the distance ${\xi }_1\gg 1$ is governed by highly connected Fock states in the space of noninteracting eigenfunctions. The resulting dynamics is subdiffusive, and the second moment grows as $m_2\sim t^{1/2}$, precisely as in the strong chaos regime for corresponding nonlinear wave equations. This surprising outcome stems from the huge Fock connectivity and resulting quantum chaos. The TIP expansion finally slows down towards a complete halt—in contrast to the nonlinear case.