Consecutive circularly polarized optical pulses generate and rotate electron spin polarization through optical orientation and the optical Stark effect. We perform time- and magnetic-field-dependent optical pump-probe measurements on gallium arsenide and observe a variable Overhauser field growth that depends on the external magnetic field and laser wavelength. We show that the time dependence of the nuclear spin polarization can be attributed to the time-averaged electron spin polarization produced along the external magnetic field direction.

We investigated plasma excitations in a disk-shaped two-dimensional electron system (2DES) laterally screened by a coplanar metal electrode surrounding the 2DES with a fixed-size separation gap around the perimeter of the disk. We find that lateral screening leads to a significant redshift in the plasmon frequency as the gap width approaches zero. It is caused by the redistribution of charges in the metallic gate that induce a logarithmic screening potential in the 2DES region. The spectroscopy of laterally screened plasmons provides valuable information on the structure of the 2DES edge. We developed a theoretical model and performed numerical simulations which show good agreement with the experimental data.

Interedge van der Waals interactions between coplanar two-dimensional metals or semimetals are investigated via the coupled-plasmon approach and many-body dispersion theory. Unlike the “standard” power law $E\sim d^{-3}$ derived from pairwise theory, the nonretarded zero-point energy does not scale with interedge distance as a power law, with the slope of the ln(−$E$)-ln($d$) relation changing from $\sim -1$ at $2\AA$ to $\sim -3$ at $10\AA$. The attractive force for two 10 nm long parallel edges of graphene separated by $4\AA$ can be up to 6.8 nN, implying that the interedge dispersion force at subnanometer separations might be experimentally detectable. We further show that the interaction can be partly screened by dielectric environments, with the strength of screening being sensitive to whether both surfaces are covered and decaying with the interlayer separation from dielectric substrates.

We use scanning tunneling microscopy to study the temperature evolution of the atomic-scale properties of the nearly commensurate charge density wave (NC-CDW) state of the low-dimensional material $1T\text{−}{\mathrm{TaS}}_2$. Our measurements at 203, 300, and 354 K, roughly spanning the temperature range of the NC-CDW state, show that while the average CDW periodicity is temperature independent, domaining and the local evolution of the CDW lattice within a domain are temperature dependent. Further, we characterize the temperature evolution of the displacement field associated with the recently discovered intradomain chirality of the NC-CDW state by calculating the local rotation vector. Intradomain chirality throughout the NC-CDW phase is likely driven by a strong coupling of the CDW lattice to the atomic lattice.

We derive bounds to the thermodynamic uncertainty relations in the linear-response regime for steady-state transport in two-terminal systems when time reversal symmetry is broken. We find that such bounds are different for charge and heat currents and depend on the details of the system, through the Onsager coefficients, and on the ratio between applied voltage and temperature difference. As a function of such a ratio, the bounds can take any positive values. The bounds are then calculated for a hybrid coherent superconducting system using the scattering approach, and the concrete case of an Andreev interferometer is explored. Interestingly, we find that the bound on the charge current is always smaller than 2 when the system operates as a heat engine, while the bound on the heat current is always larger than 2 when the system operates as a refrigerator.

Physical mechanisms of structural transformations in deposited metallic clusters exposed to an electron beam of a transmission electron microscope (TEM) are studied theoretically and computationally. Recent TEM experiments with size-selected ${\mathrm{Au}}_{923}$ clusters softly deposited on a carbon substrate showed that the clusters undergo structural transformations from icosahedron to decahedron and face-centered cubic (fcc) structures upon exposure to a 200-keV electron beam. In this paper, we demonstrate that the relaxation of collective electronic (plasmon) excitations formed in deposited metal clusters can induce the experimentally observed structural transformations. Such excitations in the clusters are formed mainly due to the interaction with low-energy secondary electrons emitted from a substrate. The characteristic occurrence times for plasmon-induced energy relaxation events are several orders of magnitude shorter than those for the momentum transfer events by energetic primary electrons to atoms of the cluster. The theoretical analysis is complemented by molecular dynamics simulations, which show that an icosahedral ${\mathrm{Au}}_{923}$ cluster softly deposited on graphite is transformed into an fcc-like structure due to the vibrational excitation of the cluster.

Scanning tunneling microscopy (STM) and transport measurements have been performed to investigate the electronic structure and its temperature dependence in heavily Sr and Na codoped PbTe, which is recognized as one of the most promising thermoelectric (TE) materials. Our main findings are as follows: (i) Below $T=4.5$ K, all carriers are distributed in the first valence band at the $L$ point $(L$ band), which forms tube-shaped Fermi surfaces with concave curvature. With Sr and Na doping, the dispersion of the $L$ band changes, and the band gap increases from $E_G\cong 200$ to $300\mathrm{meV}$. (ii) At $T=4.5$ K, the Fermi energy is located $\sim 100$ meV below the edge of the $L$ band for the Sr/Na codoped PbTe. The second valence band at the $\mathrm{\Sigma }$ point $(\mathrm{\Sigma }$ band) is lower than the $L$ band by $\mathrm{\Delta }{E}_{L\mathrm{\Sigma }}\cong 150$ meV, which is significantly smaller than that of pristine PbTe $(\mathrm{\Delta }{E}_{L\mathrm{\Sigma }}\cong 200$ meV). The decrease in $\mathrm{\Delta }{E}_{L\mathrm{\Sigma }}$, leading to band convergence, provides a desirable condition for TE materials. (iii) With increasing temperature, the carrier distribution to the $\mathrm{\Sigma }$ band starts at $T\cong 100$ K, and we estimate that $\sim 50\%$ of the total carriers are redistributed in the $\mathrm{\Sigma }$ band at $T=300$ K. In this paper, we demonstrate that STM and angular-dependent magnetoresistance measurements are particularly powerful tools to determine the electronic structure and carrier distribution. We believe that they will provide a bird's eye view of the doping strategy toward realizing high-efficiency TE materials.

The spatiotemporal dynamics of energy carriers in silicon is studied under ultrafast laser excitation. A subpicosecond temporal resolution and precise, 10 nm spatial resolution system is used to study the spatiotemporal optical response of silicon. Two distinct sets of temporal information from a single experiment are obtained: the evolution of the peak amplitude and the spatial width of the optical reflectance trace. A modified two-temperature energy transport model incorporating the changes of carrier density under ultrafast laser excitation is used to describe the underlying carrier relaxation, carrier-phonon interaction, and energy diffusion process, and is integrated into an optical model to extract fundamental energy transport dynamics and properties. By tracking the evolution of the peak amplitude change and the spatial width with the high accuracy spatiotemporal pump probe system, the underlying relaxation rates and diffusion coefficients can be simultaneously determined. In this work, the ambipolar diffusion coefficient and the carrier-phonon scattering rate coefficient of silicon are found to be $7.3{\mathrm{cm}}^2{\mathrm{s}}^{-1}$ and $3\times {10}^{11}{\mathrm{s}}^{-1}{\mathrm{K}}^{-1}$, respectively. Additionally, an ambipolar Auger recombination coefficient in the range of ${10}^{-31}-{10}^{-30}{\mathrm{cm}}^6{\mathrm{s}}^{-1}$ is obtained. This method will be useful to determine transport dynamics and properties in many newly developed semiconductor materials.

We consider bound states in the continuum (BICs) in grating composed of infinitely long silicon rods of rectangular cross-section. We reveal merging off-$\mathrm{\Gamma }$ Friedrich-Wintgen BIC with symmetry protected BIC. We present CMT and multipole decomposition theory, complementing each other, to analyze the merging phenomenon. The theories show a crossover of the behavior of $Q$ factor from standard inverse square law $k_{x,z}^{-2}$ towards extremely fast boosting law $k_{x,z}^{-6}$ in momentum space. In turn that crossover gives rise to another crossover from $Q\sim N^2$ to $Q\sim N^3$ for symmetry protected quasi-BIC in finite grating of $N$ rods owing to suppression of radiation leakage of quasi-BIC mode from surface of grating. As a result, the $Q$ factor of quasi-BIC is determined by residual leakage from ends of grating. We show numerically that this leakage can also be suppressed considerably if grating is stretched from the ends.

We theoretically study the Faraday and Kerr rotation of a probe field due to the orbital magnetization of a two-dimensional electron gas induced by a circularly polarized pump. We develop a microscopic theory of these effects in the intraband spectral range based on the analytical solution of the kinetic equation for linear and parabolic energy dispersion of electrons and arbitrary scattering potential. We show that the spectral dependence of rotation angles and accompanying ellipticities experiences a sharp resonance when the probe and pump frequencies are close to each other. At the resonance, the Faraday and Kerr rotation angles are of the order of $0.1^{\circ } \text{per} 1 \mathrm{kW}/{\mathrm{cm}}^2$ of the pump intensity in monolayer and bilayer graphene samples, corresponding to a pump-induced synthetic magnetic field of about 0.1 T. We also analyze the influence of the dielectric contrast between dielectric media surrounding the two-dimensional electron gas on the rotation angles.

The chiral geometry of the organic molecule is considered to be crucial to the chiral-induced spin selectivity (CISS) effect. However, the fundamental mechanism responsible for CISS is still debated. In this paper, we propose a mechanism of chirality transfer from chiral molecules to metals by interface coupling in a molecule/metal heterojunction. It is found that the transferred charges present a chiral distribution even in the achiral metal. Using a tight-binding model and tunneling theory, we focus on the induced chiral interface and the strong spin-orbit coupling of the metal, and predict a sizable CISS effect. This work provides a different perspective for us to understand the CISS effect.

In highly out-of-equilibrium states of matter, such as those induced by a pump laser, the applicability of well-established spectroscopic probes of magnetic order are called into question. Here we address the validity of x-ray absorption techniques in pump laser conditions, focusing on magnetic linear dichroism (MLD), a crucial probe of antiferromagnetic (AFM) order. We directly compute the dynamics of the square of the spin moment and compare the results to those obtained via the MLD response. For AFM FePd the agreement between these distinct routes to the magnetic moment severely degrades at pulse fluences greater than $\sim 1\mathrm{mJ}/{\mathrm{cm}}^2$, indicating a breakdown of the MLD response as an accurate probe of the transient moment. This contrasts with the MLD for ferromagnetic FePt, which reliably tracks the moment for fluences (and absorbed energies) up to an order of magnitude greater than the breakdown threshold for AFM FePd. We find the underlying microscopic reason for this to be increased laser induced excitations out of the $d$ band in AFM FePd, where this increase is made possible by the AFM pseudogap.

Recently, the planar Hall effect has attracted tremendous interest. In particular, an in-plane magnetization can induce an anomalous planar Hall effect with a $2\pi /3$ period for hexagon-warped energy bands. This effect is similar to the anomalous Hall effect resulting from an out-of-plane magnetization. However, this anomalous planar Hall effect is absent in the planar Hall experiments. Here, we explain its absence, by performing a calculation that includes not only the Berry curvature mechanism, as those in the previous theories, but also the disorder contributions. The conventional $\pi$-period planar Hall effect will occur if the mirror-reflection symmetry is broken, which buries the anomalous one. This is because the anomalous planar Hall effect is of the higher order with respect to the small $h/\left(E_F\tau \right)$, when compared to the conventional planar Hall effect, with $E_F$ being the Fermi energy and $\tau$ the relaxation time. We show that an in-plane strain can enhance the anomalous Hall conductivity and changes the period from $2\pi /3$ to $2\pi$. We propose a scheme to extract the hidden anomalous planar Hall conductivity from the experimental data. Our work will be helpful in detecting the anomalous planar Hall effect and could be generalized to understand mechanisms of the planar Hall effects in a wide range of materials.

Bidimensional crystals display unique properties of both fundamental and applied interest, with a good part of these properties being related to the topological aspects of 2D materials. Discrete quantum walks models, commonly used in the area of quantum information, are mathematical constructions in which the underlying network topology plays a fundamental role in determining the systems behavior. Here we present a complete scattering quantum walks approach to study 2D honeycomb lattice problems, the structure of paradigmatic 2D Dirac materials like graphene, germanene and silicene. The framework great flexibility relies on considering two arbitrary $3\times 3$ unitary scattering matrices ${\widehat{\mathrm{\Gamma }}}^{(\pm )}$ to describe the local dynamics in the lattice fundamental cell. From a simple analytic choice for ${\widehat{\mathrm{\Gamma }}}^{(\pm )}$, we address important aspects of 2D materials like transport characteristics. We also readily obtain analytic formulas for quantities which are commonly derived from a tight-binding approximation. Most importantly, we derive a rather general equation for the system energy bands based on the determinant of products of ${\widehat{\mathrm{\Gamma }}}^{(\pm )}$. We show that by properly setting these matrices (numerically), we get good agreements between our calculations for the $\pi$ and ${\pi }^{*}$ energy bands of the graphene, germanene and siliciene with accurate ab initio methods in the literature. We finally briefly discuss how the ${\widehat{\mathrm{\Gamma }}}^{(\pm )}$ could be computed from first principles, making the present an useful protocol to investigate 2D materials.

Ising (Zeeman-type) spin-orbit coupling (SOC) generated by in-plane inverse asymmetry has attracted considerable attention, especially in Ising superconductors and spin-valley coupling physics. However, many unconventional observations and emerging physical phenomena remain to be elucidated in low-symmetry two-dimensional materials. Here, we theoretically study the spin texture of ${\sigma }_z$ (spin angular momentum projection along $z$ ) induced by Ising SOC in monolayer $1T_d {\mathrm{WTe}}_2$. We predict Ising magnetoresistance (MR), whose resistance depends on the out-of-plane magnetic moment in the monolayer ${\mathrm{WTe}}_2$/ferromagnetic heterostructure. The Ising MR is believed to be an interesting counterpart to the well-studied spin Hall magnetoresistance. In addition, the unconventional charge-to-spin (named the Ising effect) and spin-to-charge conversion (named the inverse Ising effect) also contribute to the Ising MR. The recently observed “unconventional” spin-to-charge conversion can be well explained by the inverse Ising effect.

Bright excitons in ferromagnetic monolayers of ${\mathrm{CrI}}_3$ efficiently interact with lattice magnetization, which makes all-optical resonant magnetization control possible in this material. Using the combination of ab initio simulations within the Bethe-Salpeter approach, semiconductor Bloch equations, and Landau-Lifshitz equations, we construct a microscopic theory of this effect. By solving numerically the resulting set of coupled equations describing the dynamics of atomic spins and spins of the excitons, we demonstrate the possibility of tunable control of the macroscopic magnetization of a sample.

We investigate the magnon-magnon coupling between the ferromagnetic resonances (FMRs) from different ferromagnetic layers in ${\mathrm{Co}}_{90}{\mathrm{Zr}}_{10}$(100 nm)/$\mathrm{Ta}/{\mathrm{Fe}}_{20}{\mathrm{Ni}}_{80}$(100 nm) multilayers by using broadband ferromagnetic resonance. The strong coupling between two FMRs through interlayer exchange interaction is realized due to the fact that the thicknesses of both the ferromagnetic layers far exceed the corresponding exchange length. Moreover, we find that the coupling strength can be tuned by changing the thickness of the nonmagnetic Ta layer, revealing inversely proportional relationship between the coupling strength and the thickness of the nonmagnetic Ta layer. The coupling field and coupling efficiency can be regulated by varying the anisotropy of the ferromagnetic layers via oblique deposition. Furthermore, the coupling between FMR and perpendicular standing spin waves can also be achieved with suitable anisotropy and external magnetic field. These observations demonstrate that the ground state magnonic system provides a simple ideal platform for strong magnon-magnon coupling.