The transport of optically excited, hot electrons in heterostructures is analyzed by femtosecond, time-resolved two-photon photoelectron emission spectroscopy (2PPE) for epitaxial Au/Fe/MgO(001). We compare the temporal evolution of the 2PPE intensity upon optically pumping Fe or Au, while the probing occurs on the Au surface. In the case of Fe-side pumping, assuming independent relaxation in the Fe and Au layers, we determine the hot electron relaxation times in these individual layers by an analysis of the Au layer thickness dependence of the observed, effective electron lifetimes in the heterostructure. We show in addition that such a systematic analysis fails for the case of Au-side pumping due to the spatially distributed optical excitation density, which varies with the Au layer thickness. This paper extends a previous study [Beyazit et al., Phys. Rev. Lett. 125, 076803 (2020)] with data leading to reduced error bars in the determined lifetimes and by a nonlinear term in the Au thickness-dependent data analysis which contributes to similar Fe and Au film thicknesses.

We provide experimental and numerical evidence for thermal excitations within and among magnetic mesospins, forming artificial spin ice structures. At low temperatures, a decrease in magnetization and increase in susceptibility is observed with increasing temperature, interpreted as an onset of thermal fluctuations of the magnetic texture within the mesospins. At elevated temperatures a pronounced susceptibility peak is observed, related to thermally induced flipping of the mesospins and a collapse of the remanent state. The fluctuations, while occurring at distinct length and energyscales, are shown to be tunable by the interaction strength of the mesospins.

We show that including pairing and repulsion into the description of one-dimensional spinless fermions, as in the domain wall theory of commensurate melting or the interacting Kitaev chain, leads, for strong enough repulsion, to a line of critical points in the eight-vertex universality class terminating floating phases with emergent $U\left(1\right)$ symmetry. For nearest-neighbor repulsion and pairing, the variation of the critical exponents along the line that can be extracted from Baxter's exact solution of the $XYZ$ chain at $J_x=-J_z$ is fully confirmed by extensive density matrix renormalization group (DMRG) simulations of the entire phase diagram, and the qualitative features of the phase diagram are shown to be independent of the precise form of the interactions.

We investigate the nonlinear planar effects (NPEs) in Weyl semimetals (WSMs) starting from the semiclassical regime to the ultraquantum limit within the framework of Boltzmann transport theory incorporating Landau quantization. Based on our results, we propose quantum oscillations in the NPEs as a robust signature of the celebrated chiral anomaly (CA) in WSMs. By obtaining analytical expressions, we show that the quantum oscillations in the nonlinear regime exhibit two different periods in $B^{-1}$ ($B$ is the magnetic field) compared to the linear response regime with only one period in the inverse magnetic field. In addition, we obtain characteristic angular dependence of the CA-induced NPEs. We conclude that in light of the inconclusive sign of the CA-driven longitudinal magnetoconductance in WSMs as has been illustrated in recent theoretical work, the proposed behaviors of quantum oscillations in the nonlinear planar effects uniquely identify the existence of chiral anomaly in WSMs.

We report supercurrent transport measurements in hybrid Josephson junctions comprising semiconducting InAs nanowires with epitaxial ferromagnetic insulator EuS and superconducting Al coatings. The wires display a hysteretic superconducting window close to the coercivity, away from zero external magnetic field. Using a multi-interferometer setup, we measure the current-phase relation of multiple magnetic junctions and find an abrupt switch between $\pi$ and 0 phases within the superconducting window. We attribute the $0\text{−}\pi$ transition to the discrete flipping of the EuS domains and provide a qualitative theory showing that a sizable exchange field can polarize the junction and lead to the supercurrent reversal. Both 0 and $\pi$ phases can be realized at zero external field by demagnetizing the wire.

The atomic displacements induced by an x-ray beam of relatively low energy, $\epsilon \sim 8$ KeV, are investigated in pure boron oxide and in a set of sodium silicate glasses by means of x-ray photon correlation spectroscopy. We observe the complete x-ray induced transformation of the initial glass into a new amorphous state which remains stable under irradiation. The new phase continues to rearrange under the beam with a stretched exponential relaxation similar to the one observed with macroscopic measurements in the corresponding high-temperature supercooled liquid, suggesting that the new configuration lies in a higher energy minimum of the potential energy landscape. We investigate the temperature dependence of the observed dynamics for a specific sodium concentration and we observe a temperature dependence of the beam induced motion, which suggests that the defect creation rate is thermally activated. The radiation dose needed for the initial structural variation is sample dependent and correlates well with the number of constraints per vertex, within the framework of rigidity theory. This observation provides a quantitative tool to evaluate the efficiency of the radiolytic process in different network topologies.

An electronic theory of the laser-induced ultrafast magnetization dynamics in transition-metal alloys is presented. A many-body model Hamiltonian is considered that incorporates the fundamental electronic hopping, local Coulomb interaction, and spin-orbit coupling (SOC) on the same footing. Exact time propagations are performed on a tetrahedral cluster, from which the time dependencies of the local spin moments, orbital occupations, and single-particle energies of homogeneous systems and binary alloys are obtained. The consequences of inhomogeneities in the laser absorption and in the SOC strengths are investigated giving emphasis to the nature of spin-density transfers between the alloy components. A local perspective on the optically induced spin transfer is proposed in terms of which two main steps emerge: a dominantly local optical excitation followed by hopping-driven spin-density transfers among the different alloy components. The conjoint action of the spin-orbit interactions at the origin of local spin flips and spin-to-orbital angular-momentum transfer and the interatomic hoppings responsible for charge, spin, and energy flows between different sublattices is demonstrated. Transient and steady-state dynamical regimes are identified that result in delays in the onset of the local demagnetizations and in ultrafast redistributions of the spin. The central role of electron delocalization and electronic hopping in the spin-density redistribution and demagnetization of the different alloy components is demonstrated.

The symmetry of normal metal/ferromagnet bilayers allows spin-orbit torques (SOTs) to simultaneously have two distinct angular dependences on the magnetization direction $\widehat{\mathrm{m}}$. The most well-studied forms of SOT consist of the conventional fieldlike and dampinglike torques, which we label as “lowest-order” SOT. There are additional SOT forms associated with spin polarization different from that of the lowest-order SOT, and which contain an extra factor of $\widehat{\mathrm{m}}$ dependence. We label these as “higher-order” SOT. Understanding SOT-driven magnetization dynamics requires detailed information about the full angular dependence. In this paper, we measure both the lowest-order and higher-order angular dependences of SOTs in three types of bilayers, Pt/Co, Ta/CoFeB, and W/CoFeB, using harmonic Hall measurements. It is found that the higher-order SOT is negligible for Pt/Co and Ta/CoFeB, whereas it is dominant over the lowest-order one for W/CoFeB. Macrospin simulations show that the higher-order SOT can significantly affect the magnetization dynamics, which is qualitatively in line with SOT-induced switching experiments.

Antiferromagnetic $L{1}_0$-type MnPt is a material with relatively simple crystal and magnetic structures, recently attracting interest due to its high Néel temperature and wide usage as a pinning layer in magnetic devices. While it is experimentally well characterized, the theoretical understanding is much less developed, in part due to the challenging accuracy requirements dictated by the small underlying energy scales that govern magnetic ordering in antiferromagnetic metals. In this paper, we use density functional theory, the Korringa-Kohn-Rostoker formalism, and a Heisenberg model to establish a comprehensive theoretical description of antiferromagnetic $L{1}_0$-type MnPt, along with accuracy limits, by thoroughly comparing to available literature data. Our simulations show that the contribution of the magnetic dipole interaction to the magnetocrystalline anisotropy energy of $K_1=1.07\times {10}^6$ J/m${}^3$ is comparable in magnitude to the spin-orbit contribution. Using our result for the magnetic susceptibility of $5.25\times {10}^{-4}$, a lowest magnon frequency of about 2.02 THz is predicted, confirming THz spin dynamics in this material. From our data for electron, phonon, and magnon dispersion, we compute the individual contributions to the total heat capacity and show that the dominant term at or above 2 K arises from phonons. From the Landau-Lifshitz-Gilbert equation, we compute a Néel temperature of 990–1070 K. Finally, we quantify the magnitude of the magneto-optical Kerr effect generated by applying an external magnetic field. Our results provide insight into the underlying physics, which is critical for a deep understanding of fundamental limits of the time scale of spin dynamics, stability of the magnetic ordering, and the possibility of magneto-optical detection of collective spin motion.

We consider the properties of $T=0$ quantum phases of matter—especially superconducting and analogous spin-liquid phases—on infinite cylinders of width $L_{\perp }$ and analyze the ways in which the $L_{\perp }\to \infty$ (two-dimensional, 2d) limit is approached. This problem is interesting in its own right, but is particularly important in the context of extrapolating accessible density matrix renormalization group (DMRG) results on model strongly interacting problems to the desired 2d limit. Various methods for drawing firm conclusions about the quantum phases in 2d from relatively small $L_{\perp }$ results are illustrated.

Recently, the exciting reentrant localization transition phenomenon was found in a one-dimensional dimerized lattice with staggered quasiperiodic potentials. Usually, long-range hopping is typically important in actual physical systems. In this work, we study the effect of next-nearest neighbor hopping (NNNH) on the reentrant localization phenomenon. Due to the presence of NNNH, the broken chiral symmetry is further enhanced and the localization properties of electron states in the upper and lower bands become quite different. It is found that the reentrant localization can still persist within a range of NNNH both in Hermitian and non-Hermitian cases. Eventually, the reentrant localization disappears as the strength of NNNH increases to some extent, since the increasing NNNH weakens the dimerization of the system and destroys its competition with the quasiperiodic disorder. Our work thus reveals the effect of long-range hopping in the reentrant localization phenomenon and deepens its physical understanding.

The linear intrinsic anomalous Hall effect (IAHE) and second-order IAHE have been intensively investigated in time-reversal broken systems. However, as one of the important members of the nonlinear Hall family, the investigation of third-order IAHE remains absent due to the lack of an appropriate theoretical approach, although the third-order extrinsic AHE has been studied within the framework of first- and second-order semiclassical theory. Herein, we generalize the semiclassical theory for Bloch electrons under the uniform electric field up to the third order using the wave-packet method and based on which we predict that the third-order IAHE can also occur in time-reversal broken systems. Same as the second-order IAHE, we find the band geometric quantity, the second-order field-dependent Berry curvature arising from the second-order field-induced positional shift, plays a pivotal role in the observation of this effect. Moreover, with symmetry analysis, we find that the third-order IAHE, as the leading contribution, is supported by 15 time-reversal broken three-dimensional magnetic point groups, corresponding to a wide class of antiferromagnetic (AFM) materials. Guided by the symmetry arguments, a two-band model is chosen to demonstrate the generalized theory. Furthermore, the generalized third-order semiclassical theory depends only on the properties of Bloch bands, implying that it can also be employed to explore the IAHE in realistic AFM materials, by combining with first-principles calculations.

We report on measurements of flux and charge noise in an rf-SQUID (superconducting quantum interference device) flux qubit using macroscopic resonant tunneling (MRT). We measure rates of incoherent tunneling from the lowest energy state in the initial well to the ground and first excited states in the target well. The result of the measurement consists of two peaks. The first peak corresponds to tunneling to the ground state of the target well, and is dominated by flux noise. The second peak is due to tunneling to the excited state and is wider due to an intrawell relaxation process dominated by charge noise. We develop a theoretical model that allows us to extract information about flux and charge noise within one experimental setup. The model agrees very well with experimental data over a wide dynamic range and provides parameters that characterize charge and flux noise.

We analyze a special separable and chiral-symmetric model with a quantized quadrupole moment, extending the Benalcazar-Bernevig-Hughes model [Science 357, 61 (2017)]. Using the nested-Wilson-loop formalism, we give an exact expression for Wannier centers, sector polarizations, and quadrupole moments. These are connected to the winding numbers of the constitutive one-dimensional chains. We prove that these winding numbers can characterize the model's Wannier topology as a $\mathbb{Z}\times \mathbb{Z}$ set. These results clearly show that the quantization of the quadrupole moment can arise without additional spatial symmetry (except for translation symmetry) for the bulk. By switching from the Wannier representation to the Bloch representation, we derive an alternative expression for the bulk quadrupole moment and obtain its exact value. Combining the bulk quadrupole and edge polarizations, we analytically calculate the corner charge in a large square system and make the bulk-boundary correspondence explicit in an edge-consistent gauge. Our work reveals the relationship between zero-energy states at the boundary, charge localization, and the bulk quadrupole of the extended model.

Microwave absorption spectra of a single square of two-dimensional electrons (2DESs) have been investigated using an optical detection technique. Fundamental dipole and harmonic quadrupole plasmon modes have been identified and compared to those in the disk geometry. In the square-shaped 2DES, a strong interaction is discovered between the neighboring plasmon modes, whereas no such hybridization is observed in the disk-shaped geometry. We establish a rigid theoretical platform to analytically describe the magneto-optical response of confined two-dimensional systems. The developed theory provides a proper description of the obtained experimental results.

Strong coupling between excitons in transition metal dichalcogenides (TMDs) and quasibound states in the continuum (QBIC) has attracted much attention in recent years. However, the coupling strength is often limited due to the spatial mismatch at the location of the TMDs and the maximum field strength of the QBIC mode. Here, we report a cavity-assisted boosting of self-hybridization between excitons (X) and the QBIC mode at room temperature by embedding a two-dimensional (2D) metasurface composed of bulk ${\mathrm{WS}}_2$ into a microcavity. We demonstrate that the self-hybridized BIC-X coupling strength in this 2D metasurface can be dramatically enhanced with the assistance of a Fabry-Pérot cavity. Full wave simulations demonstrate a giant Rabi splitting up to 240 meV, which is twice as high as the QBIC-X self-hybridization in the 2D metasurface system. A coupled oscillator model containing a $3\times 3$ Hamiltonian matrix combined with a near-field analysis reveals the underlying mechanism of the greatly enlarged coupling strength: The cavity provides a strong out-of-plane field confinement and the QBIC mode concentrates in an in-plane electric field, which greatly facilitates the spatial overlap of excitons with the localized field. Importantly, we also demonstrate that the coupling strength of the hybrid system can be readily tuned by controlling the excitonic oscillator strength of the bulk TMD material. This provides a powerful approach for manipulating the self-hybridization process. We believe that the cavity-based configuration proposed in this paper can serve as a universal recipe for achieving strong light-matter interactions, thus paving the way for the design of tunable exciton-polariton photonic devices with high performance.

Superconductor-semiconductor hybrid devices are currently attracting much attention, fueled by the fact that strong spin-orbit interaction in combination with induced superconductivity can lead to exotic physics with potential applications in fault-tolerant quantum computation. The detailed nature of the spin dynamics in such systems is, however, often strongly dependent on device details and hard to access in experiment. In this paper we theoretically investigate a superconductor-normal-superconductor junction based on a two-dimensional hole gas with additional Rashba spin-orbit coupling, and we focus on the dependence of the critical current on the direction and magnitude of an applied in-plane magnetic field. We present a simple model, which allows us to systematically investigate different parameter regimes and obtain both numerical results and analytical expressions for all limiting cases. Our results could serve as a tool for extracting more information about the detailed spin physics in a two-dimensional hole gas based on a measured pattern of critical currents.

The $\alpha \text{−}{T}_3$ lattice, an interpolation model between the honeycomb lattice of $\mathrm{graphene}(\alpha =0$) and the dice $\mathrm{lattice}(\alpha =1$), undergoes a topological phase transition across $\alpha =1/\sqrt{2}$ when exposed to a circularly polarized off-resonant light. In this work, several features of the topological transition have been captured via various Berry phase mediated magnetic and thermoelectric effects. For instance, both the Berry curvature and the orbital magnetic moment associated with the flat band change their respective signs across $\alpha =1/\sqrt{2}$. The orbital magnetization varies linearly with the chemical potential in the forbidden gaps of the quasienergy spectrum. The slope of the orbital magnetization in the gap changes by one unit of $e/h$ as soon as $\alpha$ crosses the value $1/\sqrt{2}$ which is a direct manifestation of the corresponding change in Chern number by one unit. While the anomalous Nernst coefficient vanishes in the gaps, the anomalous Hall conductivity, however, gets quantized in a different manner on either side of $\alpha =1/\sqrt{2}$. The broken particle-hole symmetry for $0<\alpha <1$ offers valley-contrasting features in the thermoelectric coefficients as well as in the orbital magnetization which further open up the possibility to use the underlying driven system in the valley caloritronic applications.

Acoustic imaging techniques are powerful, nondestructive tools used to perform elastic imaging of systems. Acoustic imaging techniques have a large variety of applications in both industrial and fundamental research. Acoustic wave generation in the GHz range using picosecond acoustics has enabled the development of elastic images with nanometric in-depth resolution. However, the spatial lateral resolution for this imaging technique is limited by the laser spot size, which cannot be reduced below a couple hundred nanometers due to light diffraction. In this paper, we report the results from an approach in which we couple time-resolved pump-probe spectroscopy and atomic force microscopy. We generate GHz waves directly into an AFM probe from the top of the probe. Acoustic transmission from the probe to the sample is also achieved, which paves the way to perform subsurface imaging. Due to the nanometric radius size of the commercial tip, acoustic imaging with an improved lateral resolution could be achieved. These results are an encouraging step toward the development of a new acoustic microscopy technique.