A magnetoresistance method is proposed which extracts both the growth direction mobility and the planar mobility of an anisotropic thin film, such as a superlattice. A magnetic field which varies in magnitude and orientation is employed. Errors appearing in previous magnetotransport studies on superlattices are resolved. In particular, it is shown that the contact layers used to send currents through an active region do not have a negligible resistance, even if they are much more conductive than the active region. The technique is demonstrated on an InAs/GaSb-based structure where the growth axis to planar mobility anisotropy ratio is found to drop rapidly with increasing temperature.

Room-temperature and low-temperature (77 K) scanning tunneling spectroscopy and voltage-dependent scanning tunneling microscopy data are used to study the local electronic properties of the quasi-one-dimensional Si(557)-Au surface in real space. A gapped local electron density of states near the $\overline{\Gamma }$ point is observed at different positions of the surface, i.e., at protrusions arising from Si adatoms and step-edge atoms. Within the gap region, two distinct peaks are observed on the chain of localized protrusions attributed to Si adatoms. The energy gap widens on both types of protrusions after cooling from room temperature to $T=$ 77 K. The temperature dependence of the local electronic properties can therefore not be attributed to a Peierls transition occurring for the step edge only. We suggest that more attention should be paid to finite-size effects on the one-dimensional segments.

The edge states of a two-dimensional topological insulator are characterized by their helicity, a very remarkable property which is related to the time-reversal symmetry and the topology of the underlying system. We theoretically investigate a Hong-Ou-Mandel–type setup as a tool to probe it. Collisions of two electrons with the same spin show a Pauli dip, analogous to the one obtained in the integer quantum Hall case. Moreover, the collisions between electrons of opposite spin also lead to a dip, known as ${\mathbb{Z}}_2$ dip, which is a direct consequence of the constraints imposed by time-reversal symmetry. In contrast to the integer quantum Hall case, the visibility of these dips is reduced by the presence of the additional edge channels, and crucially depends on the properties of the quantum point contact. As a unique feature of this system, we show the possibility of three-electron interference, which leads to a total suppression of the noise independently of the point contact configuration. This is assured by the peculiar interplay between Fermi statistics and topology. This work intends to extend the domain of applicability of electron quantum optics.

We investigate the magnetic response of meta-atoms that can be fabricated by a bottom-up technique. Usually such meta-atoms consist of a dielectric core surrounded by a large number of solid metallic nanoparticles. In contrast to those meta-atoms considered thus far, here we study hollow metallic nanoparticles (shells). In doing so, we solve one of the most pertinent problems of current self-assembled metamaterials, namely, implementing meta-atoms with sufficiently large resonance strength and small absorption. Both conditions have to be met for deep subwavelength meta-atoms to obtain effectively homogeneous metamaterials which may be meaningfully described by negative material parameters. Eventually we show that by using these findings, self-assembled negative index materials come within reach.

Employing the random phase approximation we investigate the binding energy and Van der Waals (vdW) interlayer spacing between the two layers of bilayer transition metal dichalcogenides ${\mathrm{MoS}}_2$, ${\mathrm{MoSe}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$ for five different stacking patterns, and examine the stacking-induced modifications on the electronic and optical/excitonic properties within the GW approximation with a priori inclusion of spin-orbit coupling and by solving the two-particle Bethe-Salpeter equation. Our results show that for all cases, the most stable stacking order is the high symmetry $A{A}^{\prime }$ type, distinctive of the bulklike $2H$ symmetry, followed by the $AB$ stacking fault, typical of the $3R$ polytypism, which is by only 5 meV/formula unit less stable. The conduction band minimum is always located in the midpoint between K and $\Gamma$, regardless of the stacking and chemical composition. All $M{X}_2$ undergo an direct-to-indirect optical gap transition going from the monolayer to the bilayer regime. The stacking and the characteristic vdW interlayer distance mainly influence the valence band splitting at K and its relative energy with respect to $\Gamma$, as well as, the electron-hole binding energy and the values of the optical excitations.

We investigated Ag-adatom-induced resonant impurities of graphene by quantum capacitance measurement. Different from charged impurities and other conventional resonant impurities, Ag atoms form very weak covalent bonds with graphene. The Ag-adatom-induced resonant peak as measured by quantum capacitance grows more intense at cryogenic temperatures, at higher impurity concentrations, and in stronger magnetic fields, in accordance with our theoretical calculations. The appearance of resonant states and the split of the zeroth Landau level for Ag-adsorbed graphene are manifestations of the formation of a flat impurity band near the Dirac point.

The $J_{\mathrm{eff}}=\frac{1}{2}$ state is manifested in systems with large cubic crystal field splitting and spin-orbit coupling that are comparable to the on-site Coulomb interaction, $U$. $5d$ transition-metal oxides host parameters in this regime and strong evidence for this state in ${\text{Sr}}_2$${\text{IrO}}_4$, and additional iridates, has been presented. All the candidates, however, deviate from the cubic crystal field required to provide an unmixed canonical $J_{\mathrm{eff}}=\frac{1}{2}$ state, impacting the development of a robust model of this novel insulating and magnetic state. We present experimental and theoretical results that not only show ${\text{Ca}}_4$${\text{IrO}}_6$ hosts the state, but furthermore uniquely resides in the limit required for a canonical unmixed $J_{\mathrm{eff}}=\frac{1}{2}$ state.

When a spin-polarized current flows through a ferromagnetic (FM) metal, angular momentum is transferred to the background magnetization via spin-transfer torques. In antiferromagnetic (AFM) materials, however, the corresponding problem is unsolved. We derive microscopically the dynamics of an AFM system driven by spin current generated by an attached FM polarizer, and find that the spin current exerts a driving force on the local staggered order parameter. The mechanism does not rely on the conservation of spin angular momentum, nor does it depend on the induced FM moments on top the AFM background. Two examples are studied: (i) A domain wall is accelerated to a terminal velocity by purely adiabatic effect where the Walker's breakdown is avoided. (ii) Spin injection modifies the AFM resonance frequency, and spin current injection triggers spin wave instability of local moments above a threshold.

Multimillion atom simulations are performed to study stacking-angle ($\theta$) -dependent strain profiles, electronic structure, and polarization-resolved optical modes from [110]-tilted quantum dot stacks (QDSs). Our calculations reveal highly asymmetrical biaxial strain distributions for the tilted QDSs that strongly influence the confinements of hole wave functions and thereby control the polarization response. The calculated values of degree of polarizations, in good agreement with the available experimental data, predict a unique property of the tilted QDSs that the isotropic polarization response can be realized from both [110] and [$-110$] cleaved edges—a feature inaccessible from the conventional [001] QDSs. Detailed investigations of polar plots further establish that tilting the QDSs provides an additional knob to fine tune their polarization properties.

Multiple optical transitions from PbS quantum dots (QDs) in glass matrix are observed. Energy separations between them amount up to $\sim$200 meV. Instead of being due to a size distribution of QDs, they are found to be related to a splitting of the lowest exciton levels. Systematic analysis of the relaxation dynamics reveals the lifetimes of the split states ranging from sub-100-ns to $\sim$$\mu$s. Moreover, we observe excited and “intrinsic” states having a sub-100-ps and $\sim$ns lifetime, respectively. The behavior of the split structure can be modeled by a phonon-assisted relaxation mechanism. This investigation offers a conclusive interpretation for the wide distribution of experimentally observed nonequilibrium carrier lifetimes in PbS QDs.

We show that valley degeneracy in rotationally faulted multilayer graphene may be broken in the presence of a magnetic field and interlayer commensurations. This happens due to a simultaneous breaking of both time-reversal and inversion symmetries leading to a splitting of Landau levels linear in the field. Our theoretical work is motivated by an experiment [Y. J. Song et al., Nature (London) 467, 185 (2010)] on epitaxially grown multilayer graphene where such linear splitting of Landau levels was observed at moderate fields. We consider both bilayer and trilayer configurations and, although a linear splitting occurs in both cases, we show that the latter produces a splitting that is in quantitative agreement with the experiment.

One of the most important and accurate approaches for studying interacting many-electron correlated systems is the quantum renormalization group (RG) procedure, upon which we propose a concept in the framework of natural orbitals so that we can generalize the RG into general orbital space. To demonstrate its effectiveness, we apply this new approach on a quantum cluster-impurity model with four impurities in comparison with the exact diagonalization method. We also find a shortcut to dramatically improving this RG procedure. A profound impact on the cluster dynamical mean-field theory and multi-impurity/orbital Kondo problems is further discussed.

The possibility of a negative drag force on a defect in nonequilibrium polariton quantum fluids is presented. We relate this phenomenon to the selective parametric amplification of the waves scattered by the defect. This leads to the prediction that mobile defects acquire a nonzero velocity with respect to the polariton nonequilibrium fluid. We derive a direct relation between the drag force and the momentum distribution of the fluid, which allows for the experimental verification of our predictions.

Ferromagnetic heterostructures provide an ideal platform to explore the nature of spin-orbit torques arising from the interplay mediated by itinerant electrons between a Rashba-type spin-orbit coupling and a ferromagnetic exchange interaction. For such a prototypic system, we develop a set of coupled diffusion equations to describe the diffusive spin dynamics and spin-orbit torques. We characterize the spin torque and its two prominent—out-of-plane and in-plane—components for a wide range of relative strength between the Rashba coupling and ferromagnetic exchange. The symmetry and angular dependence of the spin torque emerging from our simple Rashba model is in an agreement with experiments. The spin diffusion equation can be generalized to incorporate dynamic effects such as spin pumping and magnetic damping.

We study the net chirality in the spin helix structure of Ho/Y multilayers induced by an in-plane applied magnetic field. The lifting of degeneracy of the chiral symmetry was revealed by means of polarized neutron reflectometry. Three samples of different thicknesses of the Ho and Y layers were grown by molecular-beam epitaxy. The chiral states are degenerated upon zero field cooling below the critical temperature $T_N=115\pm 3$ K. The chirality parameter $\gamma$ rises during the field cooling procedure in the field range from 0 to 1 T and saturates at a value of $0.12\pm 0.01$. The chirality appears stepwise below $T_N$ and depends weakly on temperature. The phenomenon is interpreted in terms of the Dzyaloshinskii-Moriya interaction appearing at the interface between the Ho and Y layers.

The upper critical field of ${\text{NaFe}}_{1-x}$${\text{Co}}_x$As was measured from $x=0$ to $x=0.08$, with the magnetic field applied parallel ($H_{\mathrm{c}2}^{\parallel }$) and normal ($H_{\mathrm{c}2}^{\perp }$) to the planes. The data were fitted using one-band and two-band models. The orbital and paramagnetic components of the upper critical field were extracted for $H_{\mathrm{c}2}^{\parallel }$, which we find to be strongly dominated by paramagnetic pair breaking. In the parent compound the paramagnetic limit is equal to the value expected by BCS theory. However, substitution of Fe by Co leads to an enhancement above the BCS limit by a factor of approximately 1.6. In the overdoped region, we observe a significant convex curvature in $H_{\mathrm{c}2}^{\perp }$ at low temperatures, which we attribute to the two-band nature of the superconductivity and the asymmetry between the two bands. Additionally, we discuss the behavior of critical field anisotropy, coherence length $\xi$, and the penetration depth $\lambda$.

From the photoluminescence of SiC microcrystals uniformly covering a rectangular mesa of the high transition temperature $T_c$ superconductor ${\text{Bi}}_2{\text{Sr}}_2{\text{CaCu}}_2{\text{O}}_{8+\delta }$, the local surface temperature $T\left(\mathit{r}\right)$ was directly measured during simultaneous sub-THz emission from the $N\sim {10}^3$ intrinsic Josephson junctions (IJJs) in the mesa. At high bias currents $I$ and low bath temperatures $T_{\mathrm{bath}}\lesssim 35$ K, the center of a large elliptical hot spot with $T\left(\mathit{r}\right)>T_c$ jumps dramatically with little current-voltage characteristic changes. The hot spot does not alter the ubiquitous primary and secondary emission conditions: the ac-Josephson relation and the electromagnetic cavity resonance excitation, respectively. Since the most intense sub-THz emission was observed for high $T_{\mathrm{bath}}\gtrsim 50$ K in the low $I$ bias regime where hot spots are absent, hot spots cannot provide the primary mechanisms for increasing the output power, the tunability, or promoting the synchronization of the $N$ IJJs for the sub-THz emission, but can at best coexist nonmutualistically with the emission. No $T\left(\mathit{r}\right)$ standing waves were observed.

We present a detailed ac susceptibility investigation of the fluctuation regime in the insulating cubic helimagnet ${\text{Cu}}_2$${\text{OSeO}}_3$. For magnetic fields ${\mu }_{0}H\ge 200$ mT, and over a wide temperature ($T$) range, the system behaves according to the scaling relations characteristic of the classical three-dimensional Heisenberg model. For lower magnetic fields, the scaling is preserved only at higher $T$ and becomes renormalized in a narrow-$T$ range above the transition temperature. Contrary to the well-studied case of MnSi, where the renormalization has been interpreted within the Brazovskii theory, our analysis of the renormalization at $H=0$ shows the fluctuation regime in ${\text{Cu}}_2$${\text{OSeO}}_3$ to lie closer to that expected within the Wilson-Fischer scenario.