We study the response of high-critical-current proximity Josephson junctions to a microwave excitation. Electron overheating in such devices is known to create hysteretic dc voltage-current characteristics. Here we demonstrate that it also strongly influences the ac response. The interplay of electron overheating and ac Josephson dynamics is revealed by the evolution of the Shapiro steps with the microwave drive amplitude. Extending the resistively shunted Josephson junction model by including a thermal balance for the electronic bath coupled to phonons, a strong electron overheating is obtained.

We show that a slowly decaying current pulse can lead to nearly deterministic precessional switching in the presence of noise. We consider a biaxial macrospin, with an easy axis in-plane and a hard axis out-of-plane, typical of thin film nanomagnets patterned into asymmetric shapes. Out-of-plane precessional magnetization orbits are excited with a current pulse with a component of spin polarization normal to the film plane. By numerically integrating the stochastic Landau-Lifshitz-Gilbert-Slonczewski equation we show that thermal noise leads to strong dephasing of the magnetization orbits. However, an adiabatically decreasing pulse amplitude overwhelmingly leads to magnetization reversal, with a final state dependent on the pulse polarity. We develop an analytic model to explain this phenomena and to determine the pulse decay time necessary for adiabatic magnetization relaxation and thus deterministic magnetization switching.

We clarify the conditions for the emergence of multiple-$\mathbf{Q}$ structures out of lattice and easy-axis spin anisotropy in frustrated magnets. By considering magnets whose exchange interaction has multiple global minima in momentum space, we find that both types of anisotropy stabilize triple-$\mathbf{Q}$ orderings. Moderate anisotropy leads to a magnetic-field-induced skyrmion crystal, which evolves into a bubble crystal for increasing spatial and spin anisotropy. The bubble crystal exhibits a quasicontinuous (devil's staircase) temperature-dependent ordering wave vector, characteristic of the competition between frustrated exchange and strong easy-axis anisotropy.

We report on a first principles study of the spin transfer torque (STT) induced by electric voltages across and temperature gradients through ${\mathrm{Fe}}_{0.5}{\mathrm{Co}}_{0.5}/\mathrm{MgO}/{\mathrm{Fe}}_{0.5}{\mathrm{Co}}_{0.5}$ (001) magnetic tunnel junctions (MTJs). A small amount of interfacial oxygen vacancies (OVs) has demonstrated considerable effects on the bias-dependent STT, enabling the “threshold voltage” for in-plane STT to be optimized to 0.2 V, and one sample with an asymmetric OV distribution reproduced the experimental measurements. The spin torque efficiency increases with the MgO barrier thickness until it reaches a constant value, while interfacial OVs tend to reduce the efficiency when compared with that of the perfect junction. The thermal spin torque (TST) is also affected greatly by imperfections at the interface, leading to enhancements in FeCo/MgO/FeCo junctions, whereas a weakening effect is found in Fe/MgO/Fe junctions. The TST magnitude in FeCo/MgO/FeCo MTJs is typically smaller than that of a similar MTJ with Fe electrodes.

Multiferroic materials are intensively investigated as potential candidates for a new generation of solar cells, due to the coexistence in their phases of ferroelectricity and magnetic order with low band gap. However, symmetry considerations, which determine the interplay between the magnetoelectric and photovoltaic properties of a light irradiated multiferroic crystal, are not fully taken into account in the current theory of the photovoltaic effect. Here the remarkable photovoltaic and magnetophotovoltaic effects occurring in the multiferroic phase of $\mathrm{KBiF}{\mathrm{e}}_2{\mathrm{O}}_5$ are described using a theoretical approach that takes directly into account the crystal and light beam wave symmetries.

We report a field-temperature phase diagram and an entropy map for the heavy-fermion compound ${\mathrm{CeAuSb}}_2$. ${\mathrm{CeAuSb}}_2$ orders antiferromagnetically below $T_N=6.6$ K and has two metamagnetic transitions, at 2.8 and 5.6 T. The locations of the critical end points of the metamagnetic transitions, which may play a strong role in the putative quantum criticality of ${\mathrm{CeAuSb}}_2$ and related compounds, are identified. The entropy map reveals an apparent entropy balance with Fermi-liquid behavior, implying that above the Néel transition the Ce moments are incorporated into the Fermi liquid. High-field data showing that the magnetic behavior is remarkably anisotropic are also reported.

Using the numerical renormalization group (NRG), we analyze the temperature dependence of the spectral function of a magnetic impurity described by the single-impurity Anderson model with a superconducting host. With increasing temperature the spectral weight is gradually transferred from the $\delta$ peak to the continuous subgap background, and both spectral features coexist at finite temperatures: the $\delta$ peak persists to temperatures of order $\mathrm{\Delta }$. The continuous background is due to inelastic exchange scattering of Bogoliubov quasiparticles off the impurity, and it is thermally activated since it requires a finite thermal population of quasiparticles above the gap. In the singlet regime for strong hybridization or away from the particle-hole symmetric point (charge-fluctuation regime) an additional subgap structure is observed just below the gap edges. It has thermally activated behavior with an activation energy equal to the Shiba state excitation energy.

The thermoelectric power of the two-dimensional electron system (2DES) at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface is explored below room temperature, in comparison with that of Nb-doped ${\mathrm{SrTiO}}_3$ single crystals. For the interface, we find a region below $T=50$ K where thermopower is dominated by phonon drag, whose amplitude is hugely amplified with respect to the corresponding bulk value, reaching values $\sim \mathrm{mV}$/K and above. The phonon-drag enhancement at the interface is traced back to the tight carrier confinement of the 2DES, and represents a sharp signature of strong electron-acoustic phonon coupling at the interface.

We simulate the time-dependent coherent dynamics of a spatially indirect exciton—an electron-hole pair with the two particles confined in different layers—in a GaAs coupled quantum well system. We use a unitary wave-packet propagation method taking into account in full the four degrees of freedom of the two particles in a two-dimensional system, including both the long-range Coulomb attraction and arbitrary two-dimensional electrostatic potentials affecting the electron and/or the hole separately. The method has been implemented for massively parallel architectures to cope with the huge numerical problem, showing good scaling properties and allowing evolution for tens of picoseconds. We have investigated both transient time phenomena and asymptotic time transmission and reflection coefficients for potential profiles consisting of (i) extended barriers and wells and (ii) a single-slit geometry. We found clear signatures of the internal two-body dynamics, with transient phenomena in the picosecond time scale which might be revealed by optical spectroscopy. Exact results have been compared with mean-field approaches which, neglecting dynamical correlations by construction, turn out to be inadequate to describe the electron-hole pair evolution in realistic experimental conditions.

We theoretically predict current generation in Weyl semimetals when circularly polarized light is applied. The electric field of the light can drive an effective magnetic field on the order of 10 T. For lower-frequency light, a nonequilibrium spin distribution is formed near the Fermi surface. Spin-momentum locking induces a giant electric current proportional to the effective magnetic field. In contrast, higher-frequency light realizes a quasistatic Floquet state with no induced electric current. We discuss the relevant materials and estimate the order of magnitude of the induced current.

We report a study on coherence of excitation of single quantum dots. We address the coherent excitation of biexcitons, the process that is indispensable for deterministic photon pair generation in quantum dots. Based on theoretical modeling we optimized the duration of the excitation pulse in our experiment to minimize the laser-induced dephasing and increase the biexciton-to-background single-exciton occupation probability. An additional effect of this approach is a high degree of time-bin entanglement with a concurrence of up to 0.78(6) and a 0.88(3) overlap with a maximally entangled state.

Titanium dioxide (${\mathrm{TiO}}_2$) has rich physical properties with potential implications for both fundamental physics and new applications. To date, the main focus of applied research is to tune its optical properties, which is usually done via doping and/or nanoengineering. However, understanding the role of $d$ electrons in materials and possible functionalization of $d$-electron properties are still major challenges. Herewith, within a combination of an innovative experimental technique, high-energy optical conductivity, and state-of-the-art ab initio electronic structure calculations, we report an emerging, novel resonant exciton in the deep ultraviolet region of the optical response. The resonant exciton evolves upon low-concentration Ta substitution in anatase ${\mathrm{TiO}}_2$ films. It is surprisingly robust and related to strong electron-electron and electron-hole interactions. The $d$- and $f$-orbital localization, due to Ta substitution, plays an unexpected role, activating strong electronic correlations and dominating the optical response under photoexcitation. Our results shed light on a new optical phenomenon in anatase ${\mathrm{TiO}}_2$ films and on the possibility of tuning electronic properties by Ta substitution.

We propose real-space renormalized dynamical mean field theory (rr-DMFT) to deal with large clusters in the framework of a cluster extension of the DMFT. In the rr-DMFT, large clusters are decomposed into multiple smaller clusters through a real-space renormalization. In this work, the renormalization effect is taken into account only at the lowest order with respect to the intercluster coupling, which nonetheless reproduces exactly both the noninteracting and atomic limits. Our method allows us large cluster-size calculations which are intractable with the conventional cluster extensions of the DMFT with impurity solvers, such as the continuous-time quantum Monte Carlo and exact diagonalization methods. We benchmark the rr-DMFT for the two-dimensional Hubbard model on a square lattice at and away from half filling, where the spatial correlations play important roles. Our results on the spin structure factor indicate that the growth of the antiferromagnetic spin correlation is taken into account beyond the decomposed cluster size. We also show that the self-energy obtained from the large-cluster solver is reproduced by our method better than the solution obtained directly for the smaller cluster. When applied to the Mott metal-insulator transition, the rr-DMFT is able to reproduce the reduced critical value for the Coulomb interaction comparable to the large cluster result.

We investigate the valley Zeeman energy (VZE) in monolayer ${\mathrm{MoS}}_2$ quantum rings, subjected to a magnetic flux $\mathrm{\Phi }$ only passing through a hole region enclosed by the inner circle of the ring. To gain insight on our numerical outcomes for finite two-dimensional rings, an analytic solution in the one-dimensional limit (zero ring width) is also presented. Although no magnetic field is applied inside the ring region, we observe finite VZEs. Interestingly, in contrast to the usual linear scenario, the VZE of the rings exhibits an oscillatory dependence on $\mathrm{\Phi }$ with possible vanishing valley Zeeman effect even in a nonzero magnetic flux due to Aharonov-Bohm type effect. On the other hand, within one period of oscillations the VZE increases linearly with $\mathrm{\Phi }$. Furthermore, for a given magnetic flux, the valley Zeeman effect is more pronounced in a ring with a stronger quantum confinement. Thus the VZE can be tuned by either magnetic flux or ring confinement or both of them. This opens a new route for controlling the valley Zeeman effect using a nonmagnetic means.

We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For $\mathrm{MoS}{\mathrm{e}}_2$ monolayer, we measure ${\tau }_{\mathrm{rad}}^0=1.8\pm 0.2\mathrm{ps}$ at $T=7\mathrm{K}$ that we interpret as the intrinsic radiative recombination time. Similar values are found for $\mathrm{WS}{\mathrm{e}}_2$ monolayers. Our detailed analysis suggests the following scenario: at low temperature $\left(T\lesssim 50\mathrm{K}\right)$, the exciton oscillator strength is so large that the entire light can be emitted before the time required for the establishment of a thermalized exciton distribution. For higher lattice temperatures, the photoluminescence dynamics is characterized by two regimes with very different characteristic times. First the photoluminescence intensity drops drastically with a decay time in the range of the picosecond driven by the escape of excitons from the radiative window due to exciton-phonon interactions. Following this first nonthermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and nonradiative recombination channels including exciton-exciton annihilation control the latter. Finally the temperature dependence of the measured exciton and trion dynamics indicates that the two populations are not in thermodynamical equilibrium.

$\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{F}{\mathrm{e}}_{0.5}$ (CoFe) ultrathin films of various thicknesses $\left(0.8\mathrm{nm}\le t_{\mathrm{CoFe}}\le 1.6\mathrm{nm}\right)$ have been grown by sputtering on (001) MgO single crystal or Si/${\mathrm{SiO}}_2$ substrates, using Pt as capping or buffer layers, respectively. The x-ray diffraction revealed an in-plane epitaxial (isotropic) growth of Pt on MgO (Si). Their magnetic properties have been studied by vibrating sample magnetometry and Brillouin light scattering (BLS) in the Damon-Eshbach geometry. Vibrating sample magnetometry characterizations show that films grown on MgO are in-plane magnetized, while films deposited on Si are perpendicularly magnetized for CoFe thickness below 1.4 nm. The BLS measurements reveal a pronounced nonreciprocal spin waves propagation, which increases with decreasing CoFe thickness. This nonreciprocity was attributed to an interfacial Dzyaloshinskii-Moriya interaction (DMI) induced by Pt interface with CoFe. Moreover, the DMI sign has been found to depend on the stacks order: it is positive (negative) for CoFe/Pt (Pt/CoFe). The effective thickness dependence of the DMI effective constant shows two regimes due to the degradation of the interfaces as the CoFe thickness decreases. We thus show that the magnetic dead layer should be taken into account to precisely determine the surface DMI constant $D_{\mathrm{s}}$. Therefore, for the thickest samples, the surface DMI constants are nearly opposite: $-1.27$ and $1.32\mathrm{pJ}{\mathrm{m}}^{-1}$ for Pt/CoFe and its reversed system, respectively.

We study ${\mathrm{TmB}}_4$, a frustrated magnet on the Archimedean Shastry-Sutherland lattice, through magnetization and transport experiments. The lack of anisotropy in resistivity shows that ${\mathrm{TmB}}_4$ is an electronically three-dimensional system. The magnetoresistance (MR) is hysteretic at low temperature even though a corresponding hysteresis in magnetization is absent. The Hall resistivity shows unconventional anomalous Hall effect (AHE) and is linear above saturation despite a large MR. We propose that complex structures at magnetic domain walls may be responsible for the hysteretic MR and may also lead to the AHE.

Through a combination of neutron diffraction and Landau theory we describe the spin ordering in the ground state of the quadruple perovskite manganite ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$—a magnetic multiferroic supporting an incommensurate orbital density wave that onsets above the magnetic ordering temperature, $T_{\mathrm{N}1}=90$ K. The multi-$\mathbf{k}$ magnetic structure in the ground state was found to be a nearly-constant-moment helix with modulated spin helicity, which oscillates in phase with the orbital occupancies on the ${\mathrm{Mn}}^{3+}$ sites via trilinear magneto-orbital coupling. Our phenomenological model also shows that, above $T_{\mathrm{N}2}=48$ K, the primary magnetic order parameter is locked into the orbital wave by an admixture of helical and collinear spin density wave structures. Furthermore, our model naturally explains the lack of a sharp dielectric anomaly at $T_{\mathrm{N}1}$ and the unusual temperature dependence of the electrical polarization.

Progress in describing thermodynamic phase transitions in quantum systems is obtained by noticing that the Gibbs operator $e^{-\beta H}$ for a two-dimensional (2D) lattice system with a Hamiltonian $H$ can be represented by a three-dimensional tensor network, the third dimension being the imaginary time (inverse temperature) $\beta$. Coarse graining the network along $\beta$ results in a 2D projected entangled-pair operator (PEPO) with a finite bond dimension $D$. The coarse graining is performed by a tree tensor network of isometries. The isometries are optimized variationally, taking into account full tensor environment, to maximize the accuracy of the PEPO. The algorithm is applied to the isotropic quantum compass model on an infinite square lattice near a symmetry-breaking phase transition at finite temperature. From the linear susceptibility in the symmetric phase and the order parameter in the symmetry-broken phase, the critical temperature is estimated at ${\mathcal{T}}_c=0.0606\left(4\right)J$, where $J$ is the isotropic coupling constant between $S=\frac{1}{2}$ pseudospins.