When the drive, which causes the level crossing in a qubit, is slow, the probability $P_{LZ}$ of the Landau-Zener transition is close to 1. In this regime, which is most promising for applications, the noise due to the coupling to the environment reduces the average $P_{LZ}$. At the same time, the survival probability, $1-P_{LZ}$, which is exponentially small for a slow drive, can be completely dominated by noise-induced correction. Our main message is that the effect of weak classical noise can be captured analytically by treating it as a perturbation in the Schrödinger equation. This allows us to study the dependence of the noise-induced correction to $P_{LZ}$ on the correlation time of the noise. As this correlation time exceeds the bare Landau-Zener transition time, the effect of noise becomes negligible. On the physical level, the mechanism of enhancement of the survival probability can be viewed as an absorption of the “noise quanta” across the gap. With characteristic energy of the quantum governed by the noise spectrum, the slower the noise is, the lower the number of quanta for which absorption is allowed energetically is. We consider two conventional realizations of noise: Gaussian noise and telegraph noise.

The origin of the large thermopower in ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ is complicated by correlation phenomena. To disentangle the effects from multiple interactions, we use angle-resolved photoemission to study ${\mathrm{K}}_x{\mathrm{RhO}}_2$, an isostructural analogy of ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ with large thermopower and weak electron correlation. Using the experimentally measured electronic structure, we demonstrate that the thermopower in ${\mathrm{K}}_x{\mathrm{RhO}}_2$ can be quantitatively explained within the quasiparticle framework after including an electron-phonon mass enhancement effect. Extending the analysis to the cobaltate, we find the doubling in thermopower is well accounted for by additional band renormalization from electron correlation. As such, the large thermopower emerges from the itinerant quasiparticles dressed by hierarchical electron-phonon and electron-electron interactions.

The two degenerate ground states of the anisotropic Heisenberg ($XY$) spin model of a chain of qubits (pseudospins) can encode quantum information, but their degree of protection against local perturbations is known to be only partial. We examine the properties of the system in the presence of nonlocal spin-spin interactions, possibly emerging from the quantum electrodynamics of the device. We find a phase distinct from the $XY$ phase admitting two ground states which are highly protected against all local field perturbations, persisting across a range of parameters. In the context of the $XY$ chain we discuss how the coupling between two ground states can be used to observe signatures of topological edge states in a small controlled chain of superconducting transmon qubits.

In our theoretical study where we combine a nonequilibrium Green's function approach with density functional theory we investigate branched compounds containing ferrocene moieties in both branches which, due to their metal centers, are designed to allow for asymmetry induced by local charging. In these compounds the ferrocene moieties are connected to pyridyl anchor groups either directly or via acetylenic spacers in a metaconnection, where we also compare our results with those obtained for the respective single-branched molecules with both meta- and paraconnections between the metal center and the anchors. We find a destructive quantum interference (DQI) feature in the transmission function slightly below the lowest unoccupied molecular orbital, which dominates the conductance even for the uncharged branched compound with spacer groups inserted. In an analysis based on mapping the structural characteristics of the range of molecules in our article onto tight-binding models, we identify the structural source of the DQI minimum as the through-space coupling between the pyridyl anchor groups. We also find that local charging in one of the branches changes the conductance only by about one order of magnitude, which we explain in terms of the spatial distributions of the relevant molecular orbitals for the branched compounds.

We propose a model of three-dimensional topological insulators consisting of weakly coupled electron- and hole-gas layers with Rashba spin-orbit interaction stacked along a given axis. We show that in the presence of strong electron-electron interactions the system realizes a fractional strong topological insulator, where the rotational symmetry and condensation energy arguments still allow us to treat the problem as quasi-one-dimensional with bosonization techniques. We also show that if Rashba and Dresselhaus spin-orbit interaction terms are equally strong, by doping the system with magnetic impurities, one can bring it into the Weyl semimetal phase.

Multidomain relaxor-based ferroelectric single crystals $\left(1\text{−}x\right)\mathrm{Pb}\left(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}\right){\mathrm{O}}_3\text{−}x\mathrm{PbTi}{\mathrm{O}}_3$ (PMN-PT) have extraordinarily large electromechanical properties, but the origin of their giant piezoelectric properties is still not well understood. The Landau-like phenomenological theory is a feasible tool to study domain structures and their correlation with piezoelectric effects, but so far no expansion coefficients have been measured due to the phase mixture complication involved. In this work, the Landau free-energy expansion parameters for $0.70\mathrm{Pb}\left(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}\right){\mathrm{O}}_3\text{−}0.30\mathrm{PbTi}{\mathrm{O}}_3$ (PMN-0.30PT) single crystal were determined from the temperature-dependent polarization-electric field $\left(P\text{−}E\right)$ hysteresis loops along ${\left[001\right]}_{\mathrm{C}}$ and ${\left[011\right]}_{\mathrm{C}}$ directions, and the rhombohedral $\left(R\right)$ to tetragonal $\left(T\right)$ phase-transition temperature. Using the obtained parameters, ferroelectric and dielectric properties were quantitatively calculated and compared with experiments. Good agreement was achieved in the temperature regions of $T$ and $R$ phases, but deviations were found in the cubic-phase temperature region since the contribution of polar nanoregions was not considered. In the phase-coexistence region from 73 to $93{}^{\circ }\mathrm{C}$, the polarization and dielectric constant can be quantitatively explained with the volume fractions of the coexisting $R$ and $T$ phases predicted by the canonical distribution. These obtained parameters can help theoretical studies and simulations of these relaxor-based ferroelectric single crystals to reveal the correlation between domain microstructures and the origin of giant electromechanical properties of multidomain PMN-PT single crystals.

We study the thermal conductivity tensor in an atomistic model of vulcanized cis-1,4-polyisoprene (PI) rubber via molecular dynamics simulations. Our polymer force field is based on V. A. Harmandaris et al. [J. Chem. Phys. 116, 436 (2002)], whereas the polymerization algorithm follows the description in J. Hager et al. [Macromolecules 48, 9039 (2015)]. The polymer chains are chemically cross linked via sulfur bridges of adjustable cross-link density. A volume-conserving uniaxial strain of up to 200% is applied to the systems. The widely used GROMACS simulation package is adapted to allow using the Green-Kubo approach to calculate the thermal conductivity tensor components. Our analysis of the heat flux autocorrelation functions leads to the conclusion that the thermal conductivity in PI is governed by short-lived phonon modes at low wave numbers due to deformation of the monomers along the polymer backbone. Applying uniaxial strain causes increased orientation of monomers along the strain direction, which enhances the attendant thermal conductivity component. We find an exponential increase of the conductivity in stretch direction in terms of an attendant orientation order parameter. This is accompanied by a simultaneous decline of thermal conductivity in the orthogonal directions. Increase of the cross-link density only has a weak effect on thermal conductivity in the unstrained system, even at high cross-link density. In the strained system we do observed a rising thermal conductivity in the limit of high stress. This increase is attributed to enhanced coupling between chains rather than to their orientation.

We have explored the magnetism in the nongeometrically frustrated spin-chain system $\gamma \text{−}{\mathrm{CoV}}_2{\mathrm{O}}_6$ which possesses a complex magnetic exchange network. Our neutron diffraction patterns at low temperatures $(T\le T_{\mathrm{N}}=6.6$ K) are best described by a model in which two magnetic phases coexist in a volume ratio 65(1) : 35(1), with each phase consisting of a single spin modulation. This model fits previous studies and our observations better than the model proposed by Lenertz et al. [J. Phys. Chem. C 118, 13981 (2014)], which consisted of one phase with two spin modulations. By decreasing the temperature from $T_{\mathrm{N}}$, the minority phase of our model undergoes an incommensurate-commensurate lock-in transition at $T^{*}=5.6$ K. Based on these results, we propose that phase separation is an alternative approach for degeneracy-lifting in frustrated magnets.

We present a combined numerical and experimental study of Andreev scattering from quantum turbulence in superfluid ${}^3\mathrm{He}\text{−}B$ at ultralow temperatures. We simulate the evolution of moderately dense, three-dimensional, quasiclassical vortex tangles and the Andreev reflection of thermal quasiparticle excitations by these tangles. This numerical simulation enables us to generate the two-dimensional map of local Andreev reflections for excitations incident on one of the faces of a cubic computational domain, and to calculate the total coefficient of Andreev reflection as a function of the vortex line density. Our numerical simulation is then compared with the experimental measurements probing quantum turbulence generated by a vibrating grid. We also address the question of whether the quasiclassical and ultraquantum regimes of quantum turbulence can be distinguished by their respective total Andreev reflectivities. We discuss the screening mechanisms which may strongly affect the total Andreev reflectivity of dense vortex tangles. Finally, we present combined numerical-experimental results for fluctuations of the Andreev reflection from a quasiclassical turbulent tangle and demonstrate that the spectral properties of the Andreev reflection reveal the nature and properties of quantum turbulence.

We study the effect of the correlated hopping term and the intersite Coulomb interaction term on principal features of the $d$-wave superconducting (SC) state, in both the electron- and hole-doped regimes within the $t\text{−}J\text{−}U$ model. In our analysis, we use the approach based on the diagrammatic expansion of the Gutzwiller wave function (DE-GWF), which allows us to go beyond the renormalized mean-field theory (RMFT). We show that the correlated hopping term enhances the pairing at the electron-doped side of the phase diagram. Moreover, the so-called non-BCS regime (which manifests itself by the negative kinetic energy gain at the transition to the SC phase) is narrowed down with the increasing magnitude of the correlated hopping $\sim K$. Also, the doping dependencies of the nodal Fermi velocity and Fermi momentum, as well as the average number of double occupancies, are analyzed with reference to the experimental data for selected values of the parameter $K$. For the sake of completeness, the influence of the intersite Coulomb repulsion on the obtained results is provided. Additionally, selected results concerning the Hubbard-model case are also presented. A complete model with all two-site interactions is briefly discussed in Appendix for reference.

Chiral and helical domain walls are generic defects of topological spin-triplet superconductors. We study theoretically the magnetic and transport properties of superconducting singlet-triplet-singlet heterostructure as a function of the phase difference between the singlet leads in the presence of chiral and helical domains inside the spin-triplet region. The local inversion symmetry breaking at the singlet-triplet interface allows the emergence of a static phase-controlled magnetization and generally yields both spin and charge currents flowing along the edges. The parity of the domain wall number affects the relative orientation of the interface moments and currents, while in some cases the domain walls themselves contribute to spin and charge transport. We demonstrate that singlet-triplet heterostructures are a generic prototype to generate and control nondissipative spin and charge effects, putting them in a broader class of systems exhibiting spin-Hall, anomalous Hall effects and similar phenomena. Features of the electron transport and magnetic effects at the interfaces can be employed to assess the presence of domains in chiral/helical superconductors.

Out-of-time-ordered correlation functions (OTOCs) are presently being extensively debated as quantifiers of dynamical chaos in interacting quantum many-body systems. We argue that in quantum spin and fermionic systems, where all local operators are bounded, an OTOC of local observables is bounded as well and thus its exponential growth is merely transient. As a better measure of quantum chaos in such systems, we propose, and study, the density of the OTOC of extensive sums of local observables, which can exhibit indefinite growth in the thermodynamic limit. We demonstrate this for the kicked quantum Ising model by using large-scale numerical results and an analytic solution in the integrable regime. In a generic case, we observe the growth of the OTOC density to be linear in time. We prove that this density in general, locally interacting, nonintegrable quantum spin and fermionic dynamical systems exhibits growth that is at most polynomial in time—a phenomenon, which we term weak quantum chaos. In the special case of the model being integrable and the observables under consideration quadratic, the OTOC density saturates to a plateau.

We investigate the relative stability, structural properties, and electronic structure of various modulated martensites of the magnetic shape memory alloy ${\mathrm{Mn}}_2\mathrm{NiGa}$ by means of density functional theory. We observe that the instability in the high-temperature cubic structure first drives the system to a structure where modulation shuffles with a period of six atomic planes are taken into account. The driving mechanism for this instability is found to be the nesting of the minority band Fermi surface, in a similar way to that established for the prototype system ${\mathrm{Ni}}_2\mathrm{MnGa}$. In agreement with experiments, we find 14M modulated structures with orthorhombic and monoclinic symmetries having energies lower than other modulated phases with the same symmetry. In addition, we also find energetically favorable 10M modulated structures which have not been observed experimentally for this system yet. The relative stability of various martensites is explained in terms of changes in the electronic structures near the Fermi level, affected mostly by the hybridization of Ni and Mn states. Our results indicate that the maximum achievable magnetic field-induced strain in ${\mathrm{Mn}}_2\mathrm{NiGa}$ would be larger than in ${\mathrm{Ni}}_2\mathrm{MnGa}$. However, the energy costs for creating nanoscale adaptive twin boundaries are found to be one order of magnitude higher than that in ${\mathrm{Ni}}_2\mathrm{MnGa}$.

We report on an extensive study of ZnO materials with cagelike motifs in clusters and bulk phases through structural searches using the minima hopping method. A novel putative ground state was discovered for the ${\left(\mathrm{ZnO}\right)}_{32}$ cluster with a tubelike structure, closely related to the previously reported ${\left(\mathrm{ZnO}\right)}_{24}$ ground-state cage geometry. Furthermore, the effect of ionization on the geometries and energetic ordering of $({\mathrm{ZnO})}_n$ clusters with $n=3–10,12$ was studied by directly sampling the energy landscape of the ionized system. Our results indicate that the transition from ring and planar structures to three-dimensional cages occurs at larger cluster sizes than in the neutral system. Inspired by the bottom-up design philosophy and the predominance of cagelike structures in medium-sized clusters, a search for crystalline ZnO was conducted aimed specifically at low-density polymorphs, resulting in the discovery of 57 novel metastable phases. The voids in these low-density materials closely resemble the hollow cage structures of small $({\mathrm{ZnO})}_n/({\mathrm{ZnO})}_n^{+}$ clusters with $n<16$. Analogous to clathrate materials, these voids could serve to accommodate guest atoms to tailor the materials properties for various applications.

We present experimental results of combined electron spin resonance (ESR) and nuclear magnetic resonance (NMR) measurements on single crystals of the Swedenborgite-type compounds ${\mathrm{YBaCo}}_{3}M{\mathrm{O}}_7$ ($M$=Al, Fe). The magnetic lattice of these materials can be described as stacks of strongly geometrically frustrated kagome layers built up of ${\mathrm{Co}}^{2+}$ ($S=3/2$) ions. Due to the Co-$M$ site intermixing, there are $M$-type defects in the kagome planes as well as magnetic Co ions at the interplanar positions. Previous investigations revealed large antiferromagnetic (AFM) Curie-Weiss temperatures in both compounds. Yet no AFM long-range order but a spin glass behavior at low temperatures has been observed. Using ${}^{27}\mathrm{Al}$ NMR as well as ${\mathrm{Co}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ESR spin probes that are sensitive to local magnetic properties at different crystallographic sites, we have identified two magnetic subsystems in both compounds and have studied their distinct properties. In particular in the case of ${\mathrm{YBaCo}}_3{\mathrm{AlO}}_7$, we have observed a gradual development of the spin correlations in the two-dimensional (2D) kagome planes containing nonmagnetic Al defects and the establishment of a 3D frozen glasslike spin state of the interplanar Co ions at lower temperatures. We have found that despite a strong dependence of the ordering temperature in the kagome layers on the type of the $M$ ion (magnetic or nonmagnetic), the final 3D static ground state sets in at rather similar temperatures. We argue that the peculiar spin dynamics and a disordered magnetic ground state of the studied compounds result from the interplay of strong magnetic frustration and intrinsic structural disorder arising due to the intersite mixing of Co and $M$ atoms.

Correlation between structure, electronic properties, and magnetism in ${\mathrm{Co}}_x{\mathrm{Gd}}_{1-x}$ thin amorphous films was investigated. The thickness averaged properties of covered thin films are consistent with those of in-depth homogeneous amorphous alloys. In spite of that, x-ray magnetic circular dichroism (XMCD) and photoemission measurements on as-grown films have shown signatures consistent with a lateral gradient composition and a Gd surfactant effect. A tendency of Gd atoms to migrate to the surface followed by an oxidation down to 8 Å has been evidenced. Our results further demonstrate the extreme sensitivity of the magnetic properties of ferrimagnetic alloys to their structure and local concentration.

We study, by means of the density-matrix renormalization group (DMRG) technique, the evolution of the ground state in a one-dimensional topological insulator, from the noninteracting to the strongly interacting limit, where the system can be mapped onto a topological Kondo-insulator model. We focus on a toy model Hamiltonian (i.e., the interacting “$sp$-ladder” model), which could be experimentally realized in optical lattices with higher orbitals loaded with ultracold fermionic atoms. Our goal is to shed light on the emergence of the strongly interacting ground state and its topological classification as the Hubbard $U$ interaction parameter of the model is increased. Our numerical results show that the ground state can be generically classified as a symmetry-protected topological phase of the Haldane type, even in the noninteracting case $U=0$ where the system can be additionally classified as a time-reversal ${\mathbb{Z}}_2$-topological insulator, and evolves adiabatically between the noninteracting and strongly interacting limits.

It is believed that most (perhaps all) gapped phases of matter can be described at long distances by topological quantum field theory (TQFT). On the other hand, it has been rigorously established that in 1+1d ground states of gapped Hamiltonians can be approximated by matrix product states (MPS). We show that the state-sum construction of 2d TQFT naturally leads to MPS in their standard form. In the case of systems with a global symmetry $G$, this leads to a classification of gapped phases in 1+1d in terms of Morita-equivalence classes of $G$-equivariant algebras. Nonuniqueness of the MPS representation is traced to the freedom of choosing an algebra in a particular Morita class. In the case of short-range entangled phases, we recover the group cohomology classification of SPT phases.

It has been recently predicted that the interplay between Coulomb interactions and Berry curvature can produce interesting optical phenomena in topologically nontrivial two-dimensional insulators. Here, we present a theory of the interband optical absorption for three-dimensional, doped Weyl semimetals. We find that the Berry curvature, Coulomb interactions, and the nonlinearity in the single-particle energy spectrum can together enable a light-induced valley polarization. We support and supplement our numerical results with an analytical toy model calculation, which unveils topologically nontrivial Mahan excitons with nonzero vorticity.