The spin lifetime anisotropy is an important quantity for investigating the spin relaxation mechanisms in graphene and in heterostructures of two-dimensional materials. We generalize the diffusive spin transport equations of oblique spin precession in a lateral spin valve with finite contact resistance. This yields a method to determine the spin lifetime anisotropy ratio $\xi ={\tau }_{\perp }/{\tau }_{\parallel }$, which is the ratio between lifetimes of spin polarized perpendicular and parallel to the graphene surface. By solving the steady-state Bloch equations, we show that the line shape of the oblique spin precession signal can be described with six dimensionless parameters, which can be solved analytically. We demonstrate that the anisotropic spin precession characteristics can be strongly suppressed by contact-induced spin relaxation originating from conductance mismatch between the channel material and electrodes. To extract the spin lifetime anisotropy ratio accurately, we develop a closed-form equation that includes the effect of finite contact resistance. Furthermore, we demonstrate that in the high contact resistance regime, the minimum channel length required for accurately determining the spin lifetime anisotropy for a sufficiently low external magnetic field is only determined by the diffusion coefficient of the channel material, as opposed to the spin diffusion length. Our work provides an accurate model to extract the spin lifetime anisotropy ratio from the oblique spin precession measurement and can be used to guide the device design for such measurements.

We introduce a simple model to describe the formation of heavy Weyl semimetals in noncentrosymmetric heavy fermion compounds under the influence of a parity-mixing, onsite hybridization. A key aspect of interaction-driven heavy Weyl semimetals is the development of surface Kondo breakdown, which is expected to give rise to a temperature-dependent reconfiguration of the Fermi arcs and the Weyl cyclotron orbits which connect them via the chiral bulk states. Our theory predicts a strong temperature-dependent transformation in the quantum oscillations at low temperatures. In addition to the effects of surface Kondo breakdown, the renormalization effects in heavy Weyl semimetals will appear in a variety of thermodynamic and transport measurements.

A mesoscopic device in the simple tunnel junction or quantum point contact geometry emits microwaves with remarkable quantum properties, when subjected to a sinusoidal drive in the GHz range. In particular, single and two-photon squeezing as well as entanglement in the frequency domain have been reported. By revising the photoassisted noise analysis developed in the framework of electron quantum optics, we present a detailed comparison between the cosine drive case and other experimentally relevant periodic voltages such as rectangular and Lorentzian pulses. We show that the latter drive is the best candidate in order to enhance quantum features and purity of the outgoing single and two-photon states, a noteworthy result in a quantum information perspective.

We investigate the laser-induced dynamics of electronically driven charge-density-wave (CDW) order. A comprehensive mean-field analysis of the attractive Hubbard model in the weak-coupling regime reveals ultrafast switching and ultrafast melting of the order via a nonthermal pathway. The resulting nonequilibrium phase diagram exhibits multiple distinct regimes of the order parameter dynamics upon increasing field strength, indicative of multiple dynamical phase transitions. Using an intuitive pseudospin picture, we show how the distinct dynamical regimes can be connected to the spin precession angle. We furthermore study the effects of electron-electron interactions beyond mean field to show that the main features of the phase diagram are robust against scattering or thermalization processes. For example, the nonthermal state with switched order is characterized by a particularly slow relaxation. The nonthermal phases can be stabilized by tailoring the pulse shape with optimal control theory. We also demonstrate how the dynamics allows to distinguish an electron-electron interaction driven CDW from an electron-phonon interaction driven CDW.

The elastic properties of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ in the high magnetic field region above 40 T, over a wide temperature range from 1.5 to 120 K, were systematically investigated by means of high-frequency ultrasonic measurements. The investigation was performed at high magnetic fields to better investigate the innate bare $5f$-electron properties, since the unidentified electronic thermodynamic phase of unknown origin, the so-called “hidden order” (HO), and associated hybridization of conduction and $f$ electrons $(c\text{−}f$ hybridization) are suppressed at high magnetic fields. From the three different transverse modes we find contrasting results; both the ${\mathrm{\Gamma }}_4\left(B_{2g}\right)$ and ${\mathrm{\Gamma }}_5\left(E_g\right)$ symmetry modes $C_{66}$ and $C_{44}$ show elastic softening that is enhanced above 30 T, while the characteristic softening of the ${\mathrm{\Gamma }}_3\left(B_{1g}\right)$ symmetry mode $(C_{11}-C_{12})/2$ is suppressed in high magnetic fields. These results underscore the presence of a hybridization-driven ${\mathrm{\Gamma }}_3\left(B_{1g}\right)$ lattice instability in ${\mathrm{URu}}_2{\mathrm{Si}}_2$. However, the results from this work cannot be explained by using existing crystalline electric field schemes applied to the quadrupolar susceptibility in a local $5f^2$ configuration. Instead, we present an analysis based on a band Jahn-Teller effect.

We propose magnetic second-order topological insulators (SOTIs). First, we study a three-dimensional model. It is pointed out that the previously proposed topological hinge insulator has actually surface states along the [001] direction in addition to hinge states. We gap out these surface states by introducing magnetization, obtaining a SOTI only with hinge states. The bulk topological number is the $Z_2$ index protected by the combined symmetry of the fourfold rotation and the inversion symmetry. We next study two-dimensional magnetic SOTIs, where the corner states are robust also in the presence of the magnetization. Finally, we construct a magnetic second-order topological semimetal by layering the two-dimensional magnetic SOTIs, where hinge-arc states are robust also in the presence of the magnetization.

Two-particle Hanbury Brown–Twiss interferometry with chiral Majorana modes produces maximally entangled electron-hole pairs. We promote the electron-hole quantum number to an interferometric degree of freedom and complete the set of linear tools for single- and two-particle interferometry by introducing a key phase gate that, combined with a Mach-Zehnder, allows full electron-hole rotations. By considering entanglement witnesses built on current cross-correlation measurements, we find that the possibility of independent local-channel rotations in the electron-hole subspace leads to a significant boost of the entanglement detection power.

${}^{77}\mathrm{Se}$ NMR measurements were made on polycrystalline and single-crystalline samples to elucidate local magnetic susceptibility and magnetic excitation of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, which is proposed to undergo an exciton condensation accompanied by a structural transition at $T_{\mathrm{c}}=328\mathrm{K}$. We determine the ${}^{77}\mathrm{Se}$ Knight shift tensors for the three Se sites and analyze their anisotropy based on the site symmetry. The temperature dependence of the Knight shift is discussed on the basis of spin and orbital susceptibilities calculated for two-chain and two-dimensional three-band models. The large fraction of the Se $4p$ orbital polarization due to the mixing between Ni $3d$ and Se $4p$ orbitals is estimated from the analysis of the transferred hyperfine coupling constant. Also the nuclear spin-lattice relaxation rate $1/T_1$ is found not to show a coherent peak just below $T_{\mathrm{c}}$ and to obey the thermally activated temperature dependence with a spin gap energy of $1770\pm 40\mathrm{K}$. This behavior of $1/T_1$ monitors the exciton condensation as proposed by the theoretical study of $1/T_1$ based on the three-chain Hubbard model for the excitonic insulator.

We determine the frequency dependence of the four independent Cartesian tensor elements of the dielectric function for monoclinic symmetry ${\mathrm{Y}}_2{\mathrm{SiO}}_5$ using generalized spectroscopic ellipsometry from 40–1200 ${\mathrm{cm}}^{-1}$. Three different crystal cuts, each perpendicular to a principle axis, are investigated. We apply our recently described augmentation of lattice anharmonicity onto the eigendielectric displacement vector summation approach [A. Mock et al., Phys. Rev. B 95, 165202 (2017)], and we present and demonstrate the application of an eigendielectric displacement loss vector summation approach with anharmonic broadening. We obtain an excellent match between all measured and model-calculated dielectric function tensor elements and all dielectric loss function tensor elements. We obtain 23 $A_u$ and 22 $B_u$ symmetry long-wavelength active transverse and longitudinal optical mode parameters including their eigenvector orientation within the monoclinic lattice. We perform density functional theory calculations and obtain 23 $A_u$ symmetry and 22 $B_u$ transverse and longitudinal optical mode parameters and their orientation within the monoclinic lattice. We compare our results from ellipsometry and density functional theory and find excellent agreement. We also determine the static and above reststrahlen spectral range dielectric tensor values and find a recently derived generalization of the Lyddane-Sachs-Teller relation for polar phonons in monoclinic symmetry materials satisfied [M. Schubert, Phys Rev. Lett. 117, 215502 (2016)].

The plasmon dispersion is inherently related to the continuum of electron-hole pair excitations. Therefore, the comparison of this continuum, as derived from band structure calculations, with experimental data of plasmon dispersion, can yield direct information about the form of the occupied as well as the unoccupied band structure in the vicinity of the Fermi level. The relevance of this statement is illustrated by a detailed analysis of plasmon dispersions in quasi-one-dimensional systems combining experimental electron energy loss spectroscopy with quantitative density-functional theory (DFT) calculations. Si(557)-Au and Si(335)-Au with single atomic chains per terrace are compared with the Si(775)-Au system, which has a double Au chain on each terrace. We demonstrate that both hybridization between Si surface states and the Au chains as well as electronic correlations lead to increasing deviations from the nearly free electron picture that is suggested by a too simple interpretation of data of angular resolved photoemission (ARPES) of these systems, particularly for the double chain system. These deviations are consistently predicted by the DFT calculations. Thus also dimensional crossover can be explained.

We report on our investigation of the influence of screening across the interface on the properties of semiconducting thin films. Using EuO as a well-defined model material, layers of various thickness deposited on yttria-stabilized zirconia (100) substrates were covered half with Mg metal and half with the wide-band-gap insulator MgO. We observed that the Curie temperature for the thinnest films is significantly higher for the part which is interfaced with the metal compared to the part which is interfaced with the insulator. We infer that the proximity of a polarizable medium reduces the energies of virtual charge excitations and thus increases the effective exchange interactions, a strong effect that can be utilized systematically for the design of thin film and multilayer systems.

One of the defining features of many-body localization is the presence of many quasilocal conserved quantities. These constants of motion constitute a cornerstone to an intuitive understanding of much of the phenomenology of many-body localized systems arising from effective Hamiltonians. They may be seen as local magnetization operators smeared out by a quasilocal unitary. However, accurately identifying such constants of motion remains a challenging problem. Current numerical constructions often capture the conserved operators only approximately, thus restricting a conclusive understanding of many-body localization. In this work, we use methods from the theory of quantum many-body systems out of equilibrium to establish an alternative approach for finding a complete set of exact constants of motion which are in addition guaranteed to represent Pauli-$z$ operators. By this we are able to construct and investigate the proposed effective Hamiltonian using exact diagonalization. Hence, our work provides an important tool expected to further boost inquiries into the breakdown of transport due to quenched disorder.

Combining physical and synthetic dimensions allows a controllable realization and manipulation of high-dimensional topological states. In our work, we introduce two quasiperiodically driven one-dimensional systems which enable tunable topological energy conversion between different driving sources. Using three drives, we realize a four-dimensional quantum Hall state which allows energy conversion between two of the drives within the bulk of the one-dimensional system. With only two drives, we achieve energy conversion between the two at the edge of the chain. Both effects are a manifestation of the effective axion electrodynamics in a three-dimensional time-reversal-invariant topological insulator. Furthermore, we explore the effects of disorder and commensurability of the driving frequencies, and show the phenomena are robust. We propose two experimental platforms, based on semiconductor heterostructures and ultracold atoms in optical lattices, in order to observe the topological energy conversion.

The organic-inorganic hybrid perovskites show excellent optical and electrical properties for photovoltaic and a myriad of other optoelectronics applications. Using high-field magneto-optical measurements up to 17.5 T at cryogenic temperatures, we have studied the spin-dependent optical transitions in the prototype $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$, which are manifested in the field-induced circularly polarized photoluminescence emission. The energy splitting between left and right circularly polarized emission bands is measured to be ∼1.5 meV at 17.5 T, from which we obtained an exciton effective $g$ factor of ∼1.32. Also from the photoluminescence diamagnetic shift we estimate the exciton binding energy to be ∼17 meV at low temperature. Surprisingly, the corresponding field-induced circular polarization is “anomalous” in that the photoluminescence emission of the higher split energy band is stronger than that of the lower split band. This “reversed” intensity ratio originates from the combination of long electron spin relaxation time and hole negative $g$ factor in $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$, which are in agreement with a model based on the k·p effective-mass approximation.

We report a theoretical study of core-shell dipolar interaction effects on the energy product of hard/soft ${\mathrm{SmCo}}_5@\mathrm{Fe}$ core-shell spherical nanoparticles. We show that the impact of the core-shell dipolar interaction on the shell reversal process is tunable by the values of the core diameter and shell thickness. Maximum energy product optimization requires small core diameters, and the energy product enhancement may reach more than $370\%$ for small core diameters ${\mathrm{SmCo}}_5@\mathrm{Fe}$ core-shell nanoparticles.

The exotic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states have been actively searched for experimentally since the mean-field based FFLO theories were put forward half a century ago. Here, we investigate the stability of FFLO states in the presence of pairing fluctuations. We conclude that FFLO superfluids cannot exist in continuum in three and two dimensions, due to their intrinsic instability, associated with infinite quantum degeneracy of the pairs. These results address the absence of convincing experimental observations of FFLO phases in both condensed matter and in ultracold atomic Fermi gases with a population imbalance. We predict that the true ground state has a pair momentum distribution highly peaked on an entire constant energy surface.

Resonant bar experiments have revealed that dynamic deformation induces nonlinearity in rocks. These experiments produce resonance curves that represent the response amplitude as a function of the driving frequency. We propose a model to reproduce the resonance curves with observed features that include (a) the log-time recovery of the resonant frequency after the deformation ends (slow dynamics), (b) the asymmetry in the direction of the driving frequency, (c) the difference between resonance curves with the driving frequency that is swept upward and downward, and (d) the presence of a “cliff” segment to the left of the resonant peak under the condition of strong nonlinearity. The model is based on a feedback cycle where the effect of softening (nonlinearity) feeds back to the deformation. This model provides a unified interpretation of both the nonlinearity and slow dynamics in resonance experiments. We further show that the asymmetry of the resonance curve is caused by the softening, which is documented by the decrease of the resonant frequency during the deformation; the cliff segment of the resonance curve is linked to a bifurcation that involves a steep change of the response amplitude when the driving frequency is changed. With weak nonlinearity, the difference between the upward- and downward-sweeping curves depends on slow dynamics; a sufficiently slow frequency sweep eliminates this up-down difference. With strong nonlinearity, the up-down difference results from both the slow dynamics and bifurcation; however, the presence of the bifurcation maintains the respective part of the up-down difference, regardless of the sweep rate.

We study the magnetic Compton profile (MCP) of the disordered ${\mathrm{Fe}}_{0.5}{\mathrm{Ni}}_{0.5}$ and of the ordered FeNi alloys and discuss the interplay between structural disorder and electronic correlations. The coherent potential approximation is employed to model the substitutional disorder within the single-site approximation, while local electronic correlations are captured with the dynamical mean field theory. Comparison with the experimental data reveals the limitation of local spin-density approximation in the low momentum region, where we show that including local but dynamic correlations the experimental spectra is excellently described. We further show that using local spin-density approximation no significant difference is seen between the MCP spectra of the disordered ${\mathrm{Fe}}_{0.5}{\mathrm{Ni}}_{0.5}$ and a hypothetical FeNi alloy having the ordered CuAu $\mathrm{L}{1}_0$ structure. Only by including the electronic correlations, the spectra significantly separate, from the second Brillouin zone boundary down to zero momenta. The difference between the MCP spectra of ordered and disordered alloys is discussed also in terms of the atomic-type decompositions. Finally based on the presented calculations we predict the shape of the MCP profile for the ordered FeNi alloy along the [111] direction.

A nonuniform temperature profile may generate a pure spin current in magnetic films, as observed, for instance, in the spin Seebeck effect. In addition, thermally induced elastic deformations may set in that could affect the spin current. A self-consistent theory of the magnonic spin Seebeck effect including thermally activated magnetoelastic effects is presented, and analytical expressions for the thermally activated deformation tensor and dispersion relations for coupled magnetoelastic modes are obtained. We derive analytical results for bulk (three-dimensional) systems and thin magnetic (two-dimensional) films. We observe that the displacement vector and the deformation tensor in bulk systems decay asymptotically as $u\sim 1/R^2$ and $\varepsilon \sim 1/R^3$, respectively, while the decays in thin magnetic films proceed slower, following $u\sim 1/R$ and $\varepsilon \sim 1/R^2$. The dispersion relations evidence a strong anisotropy in the magnetic excitations. We observe that a thermoelastic steady-state deformation may lead to both an enchantment and a reduction of the gap in the magnonic spectrum. The reduction of the gap increases the number of magnons contributing to the spin Seebeck effect and offers new possibilities for the thermoelastic control of the spin Seebeck effect.