We introduce Gutzwiller conjugate gradient minimization (GCGM) theory, an ab initio quantum many-body theory for computing the ground-state properties of infinite systems. GCGM uses the Gutzwiller wave function but does not use the commonly adopted Gutzwiller approximation (GA), which is a major source of inaccuracy. Instead, the theory uses an approximation that is based on the occupation probability of the on-site configurations, rather than approximations that decouple the site-site correlations as used in the GA. We test the theory in the one-dimensional and two-dimensional Hubbard models at various electron densities and find that GCGM reproduces energies and double occupancies in reasonable agreement with benchmark data at a very small computational cost.

Considering lattice Hamiltonians designed using conformal field theory to have fractional quantum Hall states as ground states, we study the dynamics of one or two particles on such lattices. Examining the eigenspectrum and dynamics of the single-particle sector, we demonstrate that these Hamiltonians cannot be regarded as describing interacting particles placed on a Chern band, so that the physics is fundamentally different from fractional Chern insulators. The single-particle spectrum is shown to consist of eigenstates localized in shells which have a larger radius for larger eigenenergies. This leads to chiral dynamics along reasonably well-defined orbits, in both the single-particle and the two-particle sectors.

We propose a novel phonon resonator for hindering the thermal transport of nanowires (NWs), based on a screw threadlike helical nanowall. Results from molecular dynamic simulations reveal that the thermal conductivity and phonon transmission of the screw threadlike Si NWs continuously decrease with an increase in the period density of the helical nanowall. The reduction can reach as high as 36% for the NW with six circles of the helical nanowall, which is much larger than in the case of NWs with straight nanowalls (12%) and nanopillars (15%). This phenomenon is due to that the helical nanowall has a larger contact area with the base structure, which leads to a larger volume of the resonating substructure with a constant height and width. Phonon dispersion relations show the formation of flat bands, which confirms the occurrence of phonon resonances due to the surface screw threadlike structures. The phonon spatial distribution reveals mode localization in the helical resonator at the resonant frequency. With regard to suppressing the phonon propagation, the helical nanowall as a phonon resonator exhibits superiority over the straight nanowalls or conventional nanopillars, because (1) it may be easier to fabricate a larger contact area with the base structure and (2) it avoids the problem of nanopillars or nanowalls touching each other, which maintains the ability of generating localized modes. The obtained results provide a novel design of efficient surface phonon resonators to realize nanowires with ultralow thermal conductivity.

We outline the duality between the extraordinary magnetoresistance (EMR), observed in semiconductor-metal hybrids, and nonsymmetric gravity coupled to a diffusive $U\left(1\right)$ gauge field. The corresponding gravity theory may be interpreted as the generalized complex geometry of the semidirect product of the symmetric metric and the antisymmetric Kalb-Ramond field: ($g_{\mu \nu }+{\beta }_{\mu \nu }$). We construct the four-dimensional covariant field theory and compute the resulting equations of motion. The equations encode the most general form of EMR within a well defined variational principle, for specific lower dimensional embedded geometric scenarios. Our formalism also reveals the emergence of additional diffusive pseudocurrents for a completely dynamic field theory of EMR. The proposed equations of motion now include terms that induce geometrical deformations in the device geometry in order to optimize the EMR. This bottom-up dual description between EMR and generalized geometry/gravity lends itself to a deeper insight into the EMR effect with the promise of potentially new physical phenomena and properties.

Manipulation of magnetic domain walls via a helicity-independent laser pulse has recently been experimentally demonstrated and various physical mechanisms leading to domain wall dynamics have been discussed. Spin-dependent superdiffusive transport of hot electrons has been identified as one of the possible ways to affect a magnetic domain wall. Here, we develop a model based on superdiffusive spin-dependent transport to study the laser-induced transport of hot electrons through a smooth magnetic domain wall. We show that the spin transfer between neighboring domains can enhance ultrafast demagnetization in the domain wall. More importantly, our calculations reveal that when the laser pulse is properly focused onto the vicinity of the domain wall, it can excite sufficiently strong spin currents to generate a spin-transfer torque that can rapidly move the magnetic domain wall by several nanometers in several hundred femtoseconds, leading to a huge nonequilibrium domain wall velocity.

The rotation of the plane of polarization of light passing through a nonmagnetic material is known as natural optical activity or optical gyrotropy. The behavior of this effect in thin chiral conductors is of current interest. For example, the low-frequency limit of gyrotropy in chiral three-dimensional (3D) crystals, known as the gyrotropic magnetic effect (GME), is controlled by the orbital magnetic moment of electrons, which has been proposed to be relevant to current-induced switching in twisted bilayer graphene. We show that the GME is not limited to bulk materials but also appears for quasi-2D systems with minimal structure incorporated in the third direction. Starting from multiband Kubo formula, we derive a generic expression for GME current in quasi-2D materials induced by low-frequency light, and provide a Feynman-diagrammatic interpretation. The relations between the 2D finite layered formula and 3D bulk formula are also discussed.

We study the occurrence and the origin of ferromagnetic fluctuations in the longitudinal spin susceptibility of the $t-t^{\prime }$-Rashba-Hubbard model on the square lattice. The combined effect of the second-neighbor hopping $t^{\prime }$ and the spin-orbit coupling leads to ferromagnetic fluctuations in a broad filling region. The spin-orbit coupling splits the energy bands, leading to two Van Hove fillings, where the sheets of the Fermi surface change their topology. Between these two Van Hove fillings the model shows ferromagnetic fluctuations. We find that these ferromagnetic fluctuations originate from interband contributions to the spin susceptibility. These interband contributions only arise if there is one holelike and one electronlike Fermi surface, which is the case for fillings in between the two Van Hove fillings. We discuss implications for experimental systems and propose a test on how to identify these types of ferromagnetic fluctuations in experiments.

Artificial spin ice systems, two-dimensional arrays of interacting nanomagnets, provide a playground to directly observe competing interactions. Due to the anisotropic nature of the dipolar interactions, rotation of nanomagnets is a powerful way of tuning the interactions. In this paper, we experimentally examine the ground state transition from antiferromagnetic to ferromagnetic order triggered by this transformation. The as-grown magnetic configurations well agree with Monte Carlo simulations and ascribe a unique effective temperature independent of the rotation angle. Deviations from the theoretical ground state and behavior at transition are therefore well explained.

The spintronic properties of a palladium thin film have been investigated in the presence of hydrogen gas in cobalt/palladium bilayers. Measurements of the inverse spin Hall effect (ISHE) using cavity ferromagnetic resonance allow estimations of the spin Hall conductivity and spin diffusion length in both nitrogen and hydrogen gas atmospheres. Unwanted spin rectification effects are removed using a simple method of inverting the spin current direction with respect to the measurement setup. Absorption of hydrogen gas in the Pd layer at just $3\%$ concentration results in a reduced ISHE voltage amplitude. Fitting the ISHE voltage against the Pd layer thickness demonstrates that the spin diffusion length decreases by $20\%$ in the presence of hydrogen gas. On the other hand, the results indicate that there is no significant change in the spin Hall conductivity of Pd due to hydrogen absorption.

We compute the spin-orbit torque in a transition-metal heterostructure using Slater-Koster parametrization in the two-center tight-binding approximation and accounting for $d$ orbitals only. In this method, the spin-orbit coupling is modeled within Russel-Saunders scheme, which enables us to treat interfacial and bulk spin-orbit transport on equal footing. The two components of the spin-orbit torque, dissipative (dampinglike) and reactive (fieldlike), are computed within Kubo linear response theory. By systematically studying their thickness and angular dependence, we were able to accurately characterize these components beyond the traditional inverse spin galvanic and spin Hall effects. We find that the conventional fieldlike torque is purely interfacial. In contrast, we unambiguously demonstrate that the conventional dampinglike torque possesses both an interfacial and a bulk contribution. In addition, both fieldlike and dampinglike torques display substantial angular dependence with strikingly different thickness behaviors. While the planar contribution of the fieldlike torque decreases smoothly with the nonmagnetic metal thickness, the planar contribution of the dampinglike torque increases dramatically with the nonmagnetic metal thickness. Finally, we investigate the self-torque exerted on the ferromagnet when the spin-orbit coupling of the nonmagnetic metal is turned off. Our results suggest that the spin accumulation that builds up inside the ferromagnet can be large enough to induce magnetic excitations.

Recent pump-probe experiments on underdoped cuprates and similar systems suggest the existence of a transient superconducting state above $T_c$. This poses the question of how to reliably identify the emergence of long-range order, in particular superconductivity, out of equilibrium. We investigate this point by studying a quantum quench in an extended Hubbard model and by computing various observables, which are used to identify (quasi-)long-range order in equilibrium. Our findings imply that, in contrast to current experimental studies, it does not suffice to study the time evolution of the optical conductivity to identify superconductivity. In turn, we suggest to utilize time-resolved angle-resolved photoemission spectroscopy experiments to probe for the formation of a condensate in the two-particle channel.

Ferroelectric domain walls are boundaries between regions with different polarization orientations in a ferroelectric material. Using first-principles calculations, we characterize all different types of domain walls forming on ($11\overline{1}$), (111), and ($1\overline{1}0$) crystallographic planes in thermoelectric GeTe. We find large structural distortions in the vicinity of most of these domain walls, which are driven by polarization variations. We show that such strong strain-order parameter coupling will considerably reduce the lattice thermal conductivity of GeTe samples containing domain walls with respect to a single crystal. Our results thus suggest that domain engineering is a promising path for enhancing the thermoelectric figure of merit of GeTe.

Spin dynamics of $S=2$ triangular and honeycomb lattices realized in ${\mathrm{BaNa}}_2\mathrm{Fe}{\left[{\mathrm{VO}}_4\right]}_2$ and ${\mathrm{BaNa}}_2{\mathrm{Fe}}_{2/3}{\mathrm{Mg}}_{1/3}{\left[{\mathrm{VO}}_4\right]}_2$, respectively, are investigated by magnetization, heat capacity, and particularly ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy measurements from which spin fluctuation rates are derived. Slight distortions of the $\left[{\mathrm{FeO}}_6\right]$ polyhedra from $D_{3d}$ symmetry probe the respective anisotropy of the electronic ground state. ${\mathrm{BaNa}}_2\mathrm{Fe}{\left[{\mathrm{VO}}_4\right]}_2$ and ${\mathrm{BaNa}}_2{\mathrm{Fe}}_{2/3}{\mathrm{Mg}}_{1/3}{\left[{\mathrm{VO}}_4\right]}_2$ reveal magnetic decoupling of the underlying magnetic lattices. Down to the lowest temperature of our experiments (3.3 K) only the triangular lattice exhibits long-range order ($T_c=6.5$ K). The long-range order involves only 2/3 of the Fe sites (static, $H_{hf}\approx 190$ kOe), and 1/3 of the Fe sites fluctuate faster than the observation time window of the Mössbauer spectroscopy experiment. For the honeycomb lattice two distinct iron sites alternate. The moments oriented within the plane slow down considerably with $H_{hf}\approx 120$ kOe whereas the moments on Fe sites aligned along the $c$ axis retain a three times larger fluctuation frequency. In both cases the orthogonal moments, obtained from the orientation of the hyperfine fields with respect to the electric field gradient, couple through Dzyaloshinskii-Moriya interaction. Magnetization data for both compounds show signs of ferromagnetic alignment of the reduced moments, $M$(0 kOe) $\approx 2{\mu }_B$ per Fe. The evaluation of the specific heat data and Mössbauer spectra for the ${\mathrm{BaNa}}_2\mathrm{Fe}{\left[{\mathrm{VO}}_4\right]}_2$ supports the presence of short-range antiferromagnetic interactions below 50 K and suggests a crossover into competing ferromagnetic exchange below 10 K preceding the emerging long-range order.

We present a study on spin-superfluid transport based on an atomistic, classical spin model. Easy-plane ferro- as well as antiferromagnets are considered, which allows for a direct comparison of these two material classes based on the same model assumptions. We find a spin-superfluid transport which is robust against variations of the boundary conditions, thermal fluctuations, and dissipation modeled via Gilbert damping. Though the spin accumulation is smaller for antiferromagnets the range of the spin-superfluid transport turns out to be identical for ferro- and antiferromagnets. Finally, we calculate and explore the role of the driving frequency and especially the critical frequency, where phase slips occur and the spin accumulation breaks down.

We demonstrate the possibility to build stable Josephson $\pi$-junction stripes with exciton-polariton condensates. The stability of the $\pi$ junction between arbitrary long polariton stripes is achieved at low pumping by balancing the snaking instability with counterpropagating flows towards the junction. Not dissimilar from a dark soliton, the instability becomes relevant at high pumping leading to formation of vortex dipoles. The resulting structures can be stabilized to produce static lattices of Josephson vortices in straight and ring geometries.

Logical devices based on electrical currents are ubiquitous in modern society. However, digital logic does have some drawbacks, such as a relatively high power consumption. It is, therefore, of great interest to seek alternative means to build logical circuits that can either work as stand-alone devices or in conjunction with more traditional electronic circuits. One direction that holds great promise is the use of heat currents for logical components. In the present paper, we discuss a recent abstract proposal for a quantum thermal transistor and provide a concrete design of such a device using superconducting circuits. Using a circuit quantum electrodynamics Jaynes-Cummings model, we propose a three-terminal device that allows heat transfer from source to drain, depending on the temperature of a bath coupled at the gate modulator and show that it provides similar properties to a conventional semiconductor transistor.

We present an extended zero-field muon spin relaxation (ZF-$\mu \mathrm{SR}$) study of overdoped ${\mathrm{Bi}}_{2+x}{\mathrm{Sr}}_{2-x}{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi2212) single crystals, intended to elucidate the origin of weak quasistatic magnetism previously detected by $\mu \mathrm{SR}$ in the superconducting and normal states of optimally doped and overdoped samples. New results on heavily overdoped single crystals show a similar monotonically decreasing ZF-$\mu \mathrm{SR}$ relaxation rate with increasing temperature that persists above the pseudogap (PG) temperature $T^{*}$ and does not evolve with hole doping ($p$). Additional measurements using an ultralow-background apparatus confirm that this behavior is an intrinsic property of Bi2212, which cannot be due to magnetic order associated with the PG phase. Instead we show that the temperature-dependent relaxation rate is most likely caused by structural changes that modify the contribution of the nuclear dipole fields to the ZF-$\mu \mathrm{SR}$ signal. Our results for Bi2212 emphasize the importance of not assuming that the nuclear-dipole field contribution is independent of temperature in ZF-$\mu \mathrm{SR}$ studies of high-temperature (high-$T_c$) cuprate superconductors, and do not support a recent $\mu \mathrm{SR}$ study of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ that claims to detect magnetic order in the PG phase.

Using ab initio density functional theory, we study the lattice phase transition of quasi-one-dimensional $({\mathrm{TaSe}}_4{)}_2\mathrm{I}$. In the undistorted state, the strongly anisotropic semimetal band structure presents two nonequivalent Weyl points. In previous efforts, two possible Ta-tetramerization patterns were proposed to be associated with the low-temperature structure. Our phonon calculations indicate that the orthorhombic $F222$ CDW-I phase is the most likely ground state for this quasi-one-dimensional system. In addition, the monoclinic $C2$ CDW-II phase may also be stable according to the phonon dispersion spectrum. Since these two phases have very similar energies in our density functional theory calculations, both these Ta-tetramerization distortions likely compete or coexist at low temperatures. The semimetal-to-insulator transition is induced by a Fermi-surface-driven instability that supports the Peierls scenario, which affects the Weyl physics developed above $T_{\mathrm{CDW}}$. Furthermore, the spin-orbit coupling generates Rashba-like band splittings in the insulating charge density wave phases.

A combination of density functional theory (DFT) and an efficient calculation method based on atomistic kinetic Monte Carlo simulations (AKMC) is used to investigate the interdependence of oxygen (O) and vacancy (v) diffusion in bcc Fe and in dilute iron alloys with the substitutional atoms Y and Ti. Both O and v are considered as mobile while the substitutional atoms are assumed to be immobile. DFT is applied to determine the binding energy between O and v for different distances, the migration barriers for O in the environment of v, and the corresponding barriers of v in the vicinity of O. In agreement with previous work O and v have a very strong binding at the first-neighbor distance. On the other hand, the calculations show that the Ov pair at the sixth-neighbor distance is instable. The simultaneous jumps of both O and v compensate the lack of jump paths that would occur due to this instability. The DFT results are employed to determine the diffusion coefficient of O and v using the AKMC-based calculation method. At first a model system with fixed O and v concentrations is studied. It is found that even a small v content of some parts per million can lead to a strong reduction of the O diffusivity. A similar effect is obtained for v diffusion under the influence of O. Furthermore, investigations on the interdependence of O and v diffusion in the first phase of thermal processing of oxide dispersion strengthened iron alloys are performed, and the influence of the substitutional atoms Y and Ti is studied. A simple thermodynamic model is employed to determine the concentration of O, Y, and Ti monomers as well as the total v concentration, for a typical total content of O, Y, and Ti. These results are used in calculations of the diffusion coefficients of O and v. Not only a strong mutual dependence but also a significant influence of Y on O diffusion is found. Finally, O and v diffusivities in a system with an O content close to the thermal solubility are calculated, where the monomer and total concentrations are determined by two different thermodynamic models. Even for such a low amount of O in the alloy the diffusion coefficients differ strongly from those in perfect bcc Fe.