We analyze the rate at which quantum information encoded in zero-energy Majorana modes is lost in the presence of perturbations. We show that information can survive for times that scale exponentially with the size of the chain both in the presence of quenching and time-dependent quadratic dephasing perturbations, even when the latter have spectral components above the system's energy gap. The origin of the robust storage, namely the fact that a sudden quench affects in the same way both parity sectors of the original spectrum, is discussed, together with the memory performance at finite temperatures and in the presence of particle exchange with a bath.

We present experimental evidence for the formation of ion tracks in amorphous Si induced by swift heavy-ion irradiation. An underlying core-shell structure consistent with remnants of a high-density liquid structure was revealed by small-angle x-ray scattering and molecular dynamics simulations. Ion track dimensions differ for as-implanted and relaxed Si as attributed to different microstructures and melting temperatures. The identification and characterization of ion tracks in amorphous Si yields new insight into mechanisms of damage formation due to swift heavy-ion irradiation in amorphous semiconductors.

A novel experimental technique, resonant piezoelectric spectroscopy (RPS), has been applied to investigate polar precursor effects in highly (65$\%$) B-site ordered PbSc${}_{0.5}$Ta${}_{0.5}$O${}_3$ (PST), which undergoes a ferroelectric phase transition near 300 K. The cubic-rhombohedral transition is weakly first order, with a coexistence interval of $\sim$4 K, and is accompanied by a significant elastic anomaly over a wide temperature interval. Precursor polarity in the cubic phase was detected as elastic vibrations generated by local piezoelectric excitations in the frequency range 250–710 kHz. The RPS resonance frequencies follow exactly the frequencies of elastic resonances generated by conventional resonant ultrasound spectroscopy (RUS) but RPS signals disappear on heating beyond an onset temperature, $T_{\mathrm{onset}}$, of 425 K. Differences between the RPS and RUS responses can be understood if the PST structure in the precursor regime between $T_{\mathrm{onset}}$ and the transition point, $T_{\mathrm{trans}}=300$ K, has locally polar symmetry even while it remains macroscopically cubic. It is proposed that this precursor behavior could involve the development of a tweed microstructure arising by coupling between strain and multiple order parameters, which can be understood from the perspective of Landau theory. As a function of temperature the transition is driven by the polar displacement $P$ and the order parameter for cation ordering on the crystallographic B site $Q_{\mathrm{od}}$. Results in the literature show that, as a function of pressure, there is a separate instability driven by octahedral tilting for which the assigned order parameter is $Q$. The two mainly displacive order parameters, $P$ and $Q$, are unfavorably coupled via a biquadratic term $Q^2{P}^2$, and complex tweedlike fluctuations in the precursor regime would be expected to combine aspects of all the order parameters. This would be different from the development of polar nanoregions, which are more usually evoked to explain relaxor ferroelectric behavior, such as occurs in PST with a lower degree of B-site order.

The temperature-dependent solubility of Cu in $\alpha$-Fe and initial stages of Cu precipitation are investigated in first-principles calculations and statistical thermodynamic and kinetic modeling based on ab initio effective interactions. We demonstrate that the weakening of the phase separation tendency with increasing temperature, especially close to the magnetic phase transition, is related to the strong dependence of the “chemical” interactions on the global magnetic state. At the same time, our calculations demonstrate that the vibrational contribution obtained in the quasiharmonic approximation is relatively small for temperatures near the Curie point. The results of Monte Carlo simulations of Cu solubility and clustering are in good agreement with experimental data.

The local structure of the spinel LiRh${}_2$O${}_4$ has been studied using atomic-pair distribution function analysis of powder x-ray diffraction data. This measurement is sensitive to the presence of short Rh-Rh bonds that form due to dimerization of Rh${}^{4+}$ ions on the pyrochlore sublattice, independent of the existence of long-range order. We show that structural dimers exist in the low-temperature phase, as previously supposed, with a bond shortening of $\Delta r\sim 0.15$ Å. The dimers persist up to 350 K, well above the insulator-metal transition, with $\Delta r$ decreasing in magnitude on warming. Such behavior is inconsistent with the Fermi-surface nesting-driven Peierls transition model. Instead, we argue that LiRh${}_2$O${}_4$ should properly be described as a strongly correlated system.

Neutron and x-ray diffraction measurements are presented for powders and single crystals of ${\mathrm{CaCo}}_2{\mathrm{As}}_2$. The crystal structure is a collapsed-tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure as previously reported, but with 7(1)% vacancies on the Co sites corresponding to the composition ${\mathrm{CaCo}}_{1.86\left(2\right)}{\mathrm{As}}_2$. The thermal expansion coefficients for both the $a$ and $c$ axes are positive from 10 to 300 K. Neutron diffraction measurements on single crystals demonstrate the onset of A-type collinear antiferromagnetic order below the Néel temperature $T_{\mathrm{N}}=52$(1) K with the ordered moments directed along the tetragonal $c$ axis, aligned ferromagnetically in the ab plane and antiferromagnetically stacked along the c axis.

We report on electron spin resonance (ESR) studies of the spin relaxation in Cs${}_2$CuCl${}_4$. The main source of the ESR linewidth at temperatures $T\le 150$ K is attributed to the uniform Dzyaloshinskii-Moriya interaction. The vector components of the Dzyaloshinskii-Moriya interaction are determined from the angular dependence of the ESR spectra using a high-temperature approximation. Both the angular and temperature dependencies of the ESR linewidth have been analyzed using a self-consistent quantum-mechanical approach. In addition, analytical expressions based on a quasiclassical picture for spin fluctuations are derived, which show good agreement with the quantum-approach for temperatures $T\ge 2J/k_{\mathrm{B}}\approx 15$ K. A small modulation of the ESR linewidth observed in the $ac$ plane is attributed to the anisotropic Zeeman interaction, which reflects the two magnetically nonequivalent Cu positions.

The lack of nesting of the electron and hole Fermi-surface sheets in the Fe-based superconductor LiFeAs, with a critical temperature of 18 K, has led to questions as to whether the origin of superconductivity in this material might be different from other Fe-based superconductors. Both angle-resolved photoemission and quasiparticle interference experiments have reported fully gapped superconducting order parameters with significant anisotropy. The system is also of interest because relatively strong correlations seem to be responsible for significant renormalization of the hole bands. Here we present calculations of the superconducting gap and pairing in the random-phase approximation using Fermi surfaces derived from measured photoemission spectra. The qualitative features of the gaps obtained in these calculations are shown to be different from previous two-dimensional theoretical works and in good agreement with experiment on the main Fermi-surface pockets. We analyze the contributions to the pairing vertex thus obtained and show that the scattering processes between electron and hole pockets that are believed to dominate the pairing in other Fe-based superconductors continue to do so in LiFeAs despite the lack of nesting, leading to gaps with anisotropic $s_{\pm }$ structure. Some interesting differences relating to the enhanced $d_{xy}$ orbital content of the LiFeAs Fermi surface are noted.

Density-functional calculations are performed to study the effect of epitaxial strain on PbZrO${}_3$. We find a remarkably small energy difference between the epitaxially strained polar $R3c$ and nonpolar Pbam structures over the full range of experimentally accessible epitaxial strains $-3\%⩽\eta ⩽4\%$. While ferroelectricity is favored for all compressive strains, for tensile strains the small energy difference between the nonpolar ground state and the alternative polar phase yields a robust antiferroelectric ground state. The coexistence of ferroelectricity and antiferroelectricity observed in thin films is attributed to a combination of strain and depolarization field effects.

$\alpha$-quartz has been used prolifically in recent years as an impedance matching standard in the multimegabar regime (1 Mbar $=$ 100 GPa). This is due to the fact that above $\sim$90–100 GPa along the principal Hugoniot $\alpha$-quartz becomes reflective, and thus shock velocities can be measured to high precision using velocity interferometry. This property allows for high-precision measurements, however, the accuracy of impedance matching measurements depends upon the knowledge of both the Hugoniot and the release or reshock response of $\alpha$-quartz. Here, we present the results of several adiabatic release measurements of $\alpha$-quartz from $\sim$300–1200 GPa states along the principal Hugoniot using full density polymethylpentene (commonly known as TPX), and both $\sim$190 and $\sim$110 mg/cc silica aerogel standards. These data were analyzed within the framework of a simple, analytical model that was motivated by a first-principles molecular dynamics investigation into the release response of $\alpha$-quartz. Combined, this theoretical and experimental study provides a method to perform impedance matching calculations without the need to appeal to any tabular equation of state for $\alpha$-quartz. As an analytical model, this method allows for propagation of all uncertainty, including the random measurement uncertainties and the systematic uncertainties of the Hugoniot and release response of $\alpha$-quartz. This work establishes $\alpha$-quartz for use as a high-precision standard for impedance matching in the multimegabar regime. We also note that the experimentally validated model framework should prove to be useful in the development of wide range equations of state for silica, a major constituent in the Earth's crust and mantle. Such models are crucial for accurate simulations of high-velocity giant impacts that are thought to be prevalent in the final stages of terrestrial planet formation.

The pressure induced bcc to hcp transition in Fe has been investigated via ab initio electronic structure calculations. It is found by the disordered local moment (DLM) calculations that the temperature induced spin fluctuations result in the decrease of the energy of Burgers type lattice distortions and softening of the transverse $N$-point $T{A}_1$ phonon mode with $\left[\overline{1}10\right]$ polarization. As a consequence, spin disorder in an system leads to the increase of the amplitude of atomic displacements. On the other hand, the exchange coupling parameters obtained in our calculations strongly decrease at large amplitude of lattice distortions. This results in a mutual interrelation of structural and magnetic degrees of freedom leading to the instability of the bcc structure under pressure at finite temperature.

We investigated the voltages obtained in a thin Pt strip on a permalloy film which was subject to in-plane temperature gradients and magnetic fields. The voltages detected by thin W tips or bond wires showed a purely symmetric effect with respect to the external magnetic field which can be fully explained by the planar Nernst effect. To verify the influence of the contacts, measurements in vacuum and atmosphere were compared and gave similar results. We explain that a slightly in-plane tilted temperature gradient only shifts the field direction dependence but does not cancel out the observed effects. Additionally, the anomalous Nernst effect could be induced by using thick Au tips which generated a heat current perpendicular to the sample plane. The effect can be manipulated by varying the temperature of the Au tips. These measurements are discussed concerning their relevance in transverse spin Seebeck effect measurements.

The La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$/BiFeO${}_3$ (LSMO/BFO) heterostructure has been a model system to study the interaction between ferroic order parameters at a complex oxide interface. In this study, uniaxial magnetic anisotropy is artificially induced in LSMO thin film grown on BFO with electrically patterned stripe domains. Variable-field magnetic force microscopy is exploited to investigate the in situ magnetic switching dynamics and subsequently determine the magnetic easy axis of the LSMO thin film. Intriguingly, one-to-one correspondence between the magnetization of LSMO and the polarization of BFO is found. The observed uniaxial magnetic anisotropy is attributed to the magnetocrystalline anisotropy of the LSMO, which is induced by the shear strain of the BFO lattice, rather than the interfacial magnetic coupling which would be more naturally assumed. This finding highlights the crucial role of lattice coupling at a complex oxide interface. When multiple-order parameters come into play at the heterointerface, special care is needed to deconvolute their effects on the related physical properties.

Majorana bound states are predicted to appear as boundary states of the Kitaev model. Here we show that a $\pi$-Josephson junction, inserted in a topologically nontrivial model ring, sustains a Majorana bound state, which is robust with respect to local and nonlocal perturbations. The realistic structure could be based on a high-$T_c$ superconductor tricrystal structure, similar to the one used to spot the $d$-wave order parameter. The presence of the Majorana bound state changes the ground state of the topologically nontrivial ring in a measurable way with respect to that of a conventional one.

A chiral $p_x+i{p}_y$ superconductor on a square lattice with nearest and next-nearest hopping and pairing terms is considered. Gap closures, as various parameters of the system are varied, are found analytically and used to identify the topological phases. The phases are characterized by Chern numbers (ranging from $-3$ to 3), and (numerically) by response to introduction of weak disorder, edges, and magnetic fields in an extreme type-II limit, focusing on the low-energy modes (which presumably become zero-energy Majorana modes for large lattices and separations). Several phases are found, including a phase with Chern number 3 that cannot be thought of in terms of a single range of interaction, and a phase with Chern number 2 that may host an additional, disorder-resistant Majorana mode. The energies of the vortex quasiparticle modes were found to oscillate as vortex position varied. The spatial length scale of these oscillations was found for various points in the Chern number 3 phase which increased as criticality was approached.

We have performed detailed ${}^{57}$Fe Mössbauer spectroscopy measurements on Ba${}_{0.75}$K${}_{0.25}$Fe${}_2$As${}_2$ single-crystal mosaics showing antiferromagnetic ordering below $T_N$ $=$ 95 K with superconductivity below $T_C$ around 30 K. Analysis of the Mössbauer spectra shows a decrease in the magnetic hyperfine field but no change in the magnetic volume fraction below $T_C$. This indicates coexistence of magnetism and superconductivity in these compounds.

Algebraic and geometric mean density of states in disordered systems may reveal properties of electronic localization. In order to understand the topological phases with disorder in two dimensions, we present the calculated density of states for the disordered Bernevig-Hughes-Zhang model. The topological phase is characterized by a perfectly quantized conducting plateau, carried by helical edge states, in a two-terminal setup. In the presence of disorder, the bulk of the topological phase is either a band insulator or an Anderson insulator. Both of them can protect edge states from backscattering. The topological phases are explicitly distinguished as a topological band insulator or a topological Anderson insulator from the ratio of the algebraic mean density of states to the geometric mean density of states. The calculation reveals that the topological Anderson insulator can be induced by disorders from either a topologically trivial band insulator or a topologically nontrivial band insulator.

The electronic states of the actinide dioxides AnO${}_2$ (with An = U, Np, Pu, Am, and Cm) are investigated employing first-principles calculations within the framework of the local density approximation $+U$ ($\mathrm{LDA}+U$) approach, implemented in a full-potential linearized augmented plane-wave scheme. A systematic analysis of the An-$5f$ states is performed which provides intuitive connections between the electronic structures and the local crystalline fields of the $f$ states in the AnO${}_2$ series. Particularly the mechanisms leading to the experimentally observed insulating ground states are investigated. These are found to be caused by the strong spin-orbit and Coulomb interactions of the $5f$ orbitals; however, as a result of the different configurations, this mechanism works in distinctly different ways for each of the AnO${}_2$ compounds. In agreement with experimental observations, the nonmagnetic states of plutonium and curium dioxide are computed to be insulating, whereas those of uranium, neptunium, and americium dioxides require additional symmetry breaking to reproduce the insulator ground states, a condition which is met with magnetic phase transitions. We show that the occupancy of the An-$f$ orbitals is closely connected to each of the appearing insulating mechanisms. We furthermore investigate the detailed constitution of the noncollinear multipolar moments for transverse 3$\mathit{q}$ magnetic ordered states in UO${}_2$ and longitudinal 3$\mathit{q}$ high-rank multipolar ordered states in NpO${}_2$ and AmO${}_2$.

We discuss possible topological phase transitions in Ge-based thin films of Ge(Bi${}_x$Sb${}_{1-x}$)${}_2$Te${}_4$ as a function of layer thickness and Bi concentration $x$ using the first-principles density functional theory framework. The bulk material is a topological insulator at $x=1.0$ with a single Dirac cone surface state at the surface Brillouin zone center, whereas it is a trivial insulator at $x=0$. Through a systematic examination of the band topologies, we predict that thin films of Ge(Bi${}_x$Sb${}_{1-x}$)${}_2$Te${}_4$ with $x=0.6$, 0.8, and 1.0 are candidates for two-dimensional (2D) topological insulators, which would undergo a 2D topological phase transition as a function of $x$. A topological phase diagram for Ge(Bi${}_x$Sb${}_{1-x}$)${}_2$Te${}_4$ thin films is presented to help guide their experimental exploration.

Metamaterial engineering offers a route to combine unusual and interesting optical phenomena in ways that are rare or nonexistent in nature. As an exploration of this wide parameter space, we experimentally demonstrate strong cross-phase modulation and four-wave mixing in a chiral metamaterial, highlighting the interplay of nonlinearity and circular dichroism. Furthermore, we show that the magnitude of the nonlinear parametric interaction follows certain selection rules regarding the circular polarizations of the various interacting waves. Using a coupled-mode analysis and finite element simulations, we relate these selection rules to the metamaterial's internal symmetries as well as its circular dichroism in the linear regime.