The complete characterization of the charge transport in a mesoscopic device is provided by the full counting statistics (FCS) $P_t\left(m\right)$, describing the amount of charge $Q=me$ transmitted during the time $t$. Although numerous systems have been theoretically characterized by their FCS, the experimental measurement of the distribution function $P_t\left(m\right)$ or its moments $\langle Q^n\rangle$ is rare and often plagued by strong back-action. Here, we present a strategy for the measurement of the FCS, more specifically its characteristic function $\chi \left(\lambda \right)$ and moments $\langle Q^n\rangle$, by a qubit with a set of different couplings ${\lambda }_j$, $j=1,\cdots ,k,\cdots k+p$, $k=\lceil n/2\rceil$, $p\ge 0$, to the mesoscopic conductor. The scheme involves multiple readings of Ramsey sequences at the different coupling strengths ${\lambda }_j$, and we find the optimal distribution for these couplings ${\lambda }_j$ as well as the optimal distribution $N_j$ of $N=\sum N_j$ measurements among the different couplings ${\lambda }_j$. We determine the precision scaling for the moments $\langle Q^n\rangle$ with the number $N$ of invested resources and show that the standard quantum limit can be approached when many additional couplings $p\gg 1$ are included in the measurement scheme.

Two-dimensional transition-metal dichalcogenide materials have recently attracted great attention from the scientific community due to their interesting properties such as the presence of an energy band gap and the support of spin-polarized states. In particular, monolayer molybdenum disulfide has a structure with no inversion symmetry and, thus, presents a large spin-splitting of the top valence bands. This latter fact makes it favorable for studies of optical spin injection, a phenomenon that, under incidence of circularly polarized light, creates spin-polarized electrons in the conduction bands. Here, we perform a theoretical study of the one-photon optical spin and current injection on transition-metal dichalcogenide monolayers of molybdenum disulfide. In particular, we present calculations for spectra of the degree of spin polarization, which are calculated in a full-band structure scheme employing density functional theory; besides the so-called quasiparticle GW approximation is applied for the calculation of the band gap energy correction. Our results show 100% of spin polarization of the electrons for the one monolayer structure at the K valley. The degree of spin polarization also presents a net maximum value at the direct energy band gap at K as the number of monolayers increases.

We present a method capable of calculating elastic scanning tunneling microscopy (STM) currents from localized atomic orbital density functional theory (DFT). To overcome the poor accuracy of the localized orbital description of the wave functions far away from the atoms, we propagate the wave functions, using the total DFT potential. From the propagated wave functions, the Bardeen's perturbative approach provides the tunneling current. To illustrate the method we investigate carbon monoxide adsorbed on a Cu(111) surface and recover the depression/protrusion observed experimentally with normal/CO-functionalized STM tips. The theory furthermore allows us to discuss the significance of $s$- and $p$-wave tips.

We present a comprehensive theoretical analysis and computational study of four-wave mixing (FWM) of optical pulses co-propagating in one-dimensional silicon photonic crystal waveguides (Si-PhCWGs). Our theoretical analysis describes a very general setup of the interacting optical pulses, namely we consider nondegenerate FWM in a configuration in which at each frequency there exists a superposition of guiding modes. We incorporate in our theoretical model all relevant linear optical effects, including waveguide loss, free-carrier (FC) dispersion and FC absorption, nonlinear optical effects such as self- and cross-phase modulation (SPM, XPM), two-photon absorption (TPA), and cross-absorption modulation (XAM), as well as the coupled dynamics of free-carriers FCs and optical field. In particular, our theoretical analysis based on the coupled-mode theory provides rigorously derived formulas for linear dispersion coefficients of the guiding modes, linear coupling coefficients between these modes, as well as the nonlinear waveguide coefficients describing SPM, XPM, TPA, XAM, and FWM. In addition, our theoretical analysis and numerical simulations reveal key differences between the characteristics of FWM in the slow- and fast-light regimes, which could potentially have important implications to the design of ultracompact active photonic devices.

We present a systematic study of both the temperature and frequency dependence of the optical response in TaAs, a material that has recently been realized to host the Weyl semimetal state. Our study reveals that the optical conductivity of TaAs features a narrow Drude response alongside a conspicuous linear dependence on frequency. The weight of the Drude peak decreases upon cooling, following a $T^2$ temperature dependence, in good agreement with theoretical predictions. Two linear components with distinct slopes dominate the low-temperature optical conductivity. A comparison between our experimental results and theoretical calculations suggests that the linear conductivity below $\sim 230 {\text{cm}}^{-1}$ arises purely from interband transitions near the Weyl points, providing rich information about the Weyl semimetal state in TaAs.

We report on the effect of in-plane magnetic field $B_{\parallel }$ on stripe phases in higher ($N=2,3$) Landau levels of a high-mobility two-dimensional electron gas. In accord with previous studies, we find that a modest $B_{\parallel }$ applied parallel to the native stripes aligns them perpendicular to it. However, upon further increase of $B_{\parallel }$, stripes are reoriented back to their native direction. Remarkably, applying $B_{\parallel }$ perpendicular to the native stripes also aligns stripes parallel to it. Thus, regardless of the initial orientation of stripes with respect to $B_{\parallel }$, stripes are ultimately aligned parallel to $B_{\parallel }$. These findings provide evidence for a $B_{\parallel }$-induced symmetry-breaking mechanism which challenge current understanding of the role of $B_{\parallel }$ and should be taken into account when determining the strength of the native symmetry-breaking potential. Finally, our results might indicate nontrivial coupling between the native and external symmetry-breaking fields, which has not yet been theoretically considered.

Using high resolution x-ray diffraction (XRD) on high purity powders, we resolved the structure and $ab$ symmetry of the intriguing compound ${\mathrm{Sr}}_2{\mathrm{VO}}_4$ from room temperature down to 20 K to an unprecedented level of accuracy. Upon cooling, this set of data unambiguously reveals a second-order phase transition lowering the symmetry from tetragonal to orthorhombic at a temperature $T_{c2}=136$ K. The observation of an orthorhombic distortion of the $ab$ plane is attributed to nematic phase formation supported by local Jahn-Teller (JT) dynamical instability. At $T_N=105$ K, spins order and at $T_{c1}=100$ K the tetragonal structure is recovered with an elongated $c$ axis.

Using density functional theory (DFT) with a van der Waals density functional, we calculate the adsorption energetics and geometry of benzenediamine (BDA) molecules on Au(111) surfaces. Our results demonstrate that the reported self-assembled linear chain structure of BDA, stabilized via hydrogen bonds between amine groups, is energetically favored over previously studied monomeric phases. Moreover, using a model, which includes nonlocal polarization effects from the substrate and the neighboring molecules and incorporates many-body perturbation theory calculations within the GW approximation, we obtain approximate self-energy corrections to the DFT highest occupied molecular orbital (HOMO) energy associated with BDA adsorbate phases. We find that, independent of coverage, the HOMO energy of the linear chain phase is lower relative to the Fermi energy than that of the monomer phase, and in good agreement with values measured with ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy.

This paper is focused on the variable-range hopping of electrons in semiconductor nanocrystal (NC) films below the critical doping concentration $n_c$ at which it becomes metallic. The hopping conductivity is described by the Efros-Shklovskii law, which depends on the localization length of electrons. We study how the localization length grows with the doping concentration $n$ in the film of touching NCs. For that we calculate the electron transfer matrix element $t\left(n\right)$ between neighboring NCs for two models when NCs touch by small facets or just one point. We study two sources of disorder: variations of NC diameters and random Coulomb potentials originating from random numbers of donors in NCs. We use the ratio of $t\left(n\right)$ to the disorder-induced NC level dispersion to find the localization length of electrons due to the multistep elastic co-tunneling process. We found three different phases at $n<n_c$ depending on the strength of disorder, the material, sizes of NCs and their facets: (1) “insulator” where the localization length of electrons increases monotonically with $n$, (2) “oscillating insulator” when the localization length (and the conductivity) oscillates with $n$ from the insulator base, and (3) “blinking metal” where the localization length periodically diverges. The first two phases were seen experimentally and we discuss how one can see the more exotic third one. In all three, the localization length diverges at $n=n_c$. This allows us to find $n_c$.

Motivated by recent free-standing graphene experiments, we show how thermal fluctuations affect the mechanical properties of microscopically thin solid ribbons, which can be many thousand times wider than their atomic thickness. A renormalization group analysis of flexural phonons reveals that elongated ribbons behave like highly anisotropic polymers, where the two dimensional nature of ribbons is reflected in nontrivial power law scalings of the persistence length and effective bending and twisting rigidities with the ribbon width. With a coarse-grained transfer matrix approach, we then examine the nonlinear response of thermalized ribbons to pulling and bending forces over a wide spectrum of temperatures, forces, and ribbon lengths.

The correlation between structural and magnetic properties of $\mathrm{GdA}{\mathrm{l}}_2$, focusing on the role played by the disorder in magnetic ordering and how it influences the magnetocaloric effect (MCE) are discussed. Micrometric-sized particles, consisting of nanocrystallites embedded in an amorphous matrix, were prepared by a mechanical milling technique and characterized by means of x-ray diffraction, scanning and high-resolution transmission electron microscopy as well as magnetic measurements as a function of an applied external magnetic field and temperature. The results show that the average particle size is just slightly diminished (≈7%) with the milling time (between 3 and 13 h), whereas the average crystallite size undergoes an expressive reduction (≈43%). For long milling times, structural disorders mostly associated with crystallite size singularly affect the magnetic properties, leading to a large tablelike MCE in the temperature range between 30 and 165 K. Below 30 K, nanocrystallites with dimensions below a given critical size cause an enhancement in the magnetic entropy change related to superparamagnetic behavior. In contrast, for low milling times, relative cooling power values are improved. These striking features along with the small magnetic hysteresis observed make the milled $\mathrm{GdA}{\mathrm{l}}_2$ a promising material for application in the magnetic refrigeration technology. Finally, a discussion in an attempt to elucidate the origin of the spin-glass states previously reported in the literature for mechanically milled $\mathrm{GdA}{\mathrm{l}}_2$ samples for very long times (400 and 1000 h) is presented.

We study the spin and charge currents flowing at the interface of an itinerant ferromagnet with a topological spin-triplet superconductor having a different number of time-reversal-invariant Majorana helical modes. Depending on the number of helical modes, the capacity of carrying spin and charge currents is shown to be directly related to the amplitude and orientation of the ferromagnetic magnetization with respect to the superconducting $\vec{d}$ vector. Differently from the one-helical mode spin-triplet superconductor, we find that the presence of a finite amount of electronic hybridization with the two pairs of Majorana helical modes leads to nonvanishing charge current independently of the ferromagnetic exchange. The competition between the two pairs of Majorana helical modes remarkably yields a spin-current response that is almost constant in the range of weak to intermediate ferromagnetism. The behavior of the spin current is tightly linked to the direction of the spin polarization in the ferromagnet and tends to be flatter for a magnetization that is coplanar to the spin-triplet $\vec{d}$ vector independently of the number of helical modes. We also investigate how the currents response is altered due to charge- and spin-density inhomogeneities at the interface as well as in response to a spin-polarizing field in the interior of the topological superconductor.

Despite having the simplest atomic structure, bulk FeSe has an observed electronic structure with the largest deviation from the band theory predictions among all Fe-based superconductors and exhibits a low-temperature nematic electronic state without intervening magnetic order. We show that the Fe-Fe interatomic Coulomb repulsion $V$ offers a natural explanation for the puzzling electron correlation effects in FeSe superconductors. It produces a strongly renormalized low-energy band structure where the van Hove singularity sits remarkably close to Fermi level in the high-temperature electron liquid phase as observed experimentally. This proximity enables the quantum fluctuations in $V$ to induce a rotational symmetry breaking electronic bond order in the $d$-wave channel. We argue that this emergent low-temperature $d$-wave bond nematic state, different from the commonly discussed ferro-orbital order and spin nematicity, has been observed recently by several angle-resolved photoemission experiments detecting the lifting of the band degeneracies at high-symmetry points in the Brillouin zone. We present a symmetry analysis of the space group and identify the hidden antiunitary $T$ symmetry that protects the band degeneracy and the electronic order/interaction that can break the symmetry and lift the degeneracy. We show that the $d$-wave nematic bond order, together with the spin-orbit coupling, provide a unique explanation of the temperature dependence, momentum space anisotropy, and domain effects observed experimentally. We discuss the implications of our findings on the structural transition, the absence of magnetic order, and the intricate competition between nematicity and superconductivity in FeSe superconductors.

Microcavity polaritons in the lasing regime undergo a spontaneous symmetry-breaking transition resulting in coherent emission with a well-defined polarization. The order parameter is thus a vector describing both the laser global phase and its polarization. Using an ultrafast single-shot detection technique, we show that polariton lasing in GaAs-based microcavities presents a high degree of second-order coherence $[g^{\left(2\right)}(\tau =0)\approx 1]$ above threshold, and that the initial polarization is stochastic, taking any possible direction in the Poincaré sphere (linear, elliptical, or circular). Once the polarization direction is established, subsequent oscillations of the emission probability witness the presence of an intrinsic polarization splitting. These results show the intricate polarization dynamics in the onset of polariton lasers.

We theoretically investigate the supercurrent-induced magnetization dynamics of a two-dimensional lattice of ferromagnetically ordered spins placed on a conventional superconductor with broken spatial inversion symmetry and strong spin-orbit coupling. We develop a phenomenological description of the coupled dynamics of the superconducting condensate and the spin system and demonstrate that supercurrents produce a reactive spin-orbit torque on the magnetization. By performing a microscopic self-consistent calculation, we show that the spin-orbit torque originates from a spin polarization of the Cooper pairs due to current-induced spin-triplet correlations. Interestingly, we find that there exists an intrinsic limitation for the maximum achievable spin-orbit torque, which is determined by the coupling strength between the condensate and the spin system. In proximitized hole-doped semiconductors, the maximum achievable spin-orbit torque field is estimated to be on the order of $0.16$ mT, which is comparable to the critical field for current-induced magnetization switching in ferromagnetic semiconductors.

We propose a transport blockade mechanism in quantum dot arrays and conducting molecules based on an interplay of Coulomb repulsion and the formation of edge states. As a model we employ a dimer chain that exhibits a topological phase transition. The connection to a strongly biased electron source and drain enables transport. We show that the related emergence of edge states is manifest in the shot noise properties as it is accompanied by a crossover from bunched electron transport to a Poissonian process. For both regions we develop a scenario that can be captured by a rate equation. The resulting analytical expressions for the Fano factor agree well with the numerical solution of a full quantum master equation.

A codimension-one critical surface in momentum space can be either a familiar Fermi surface, which separates occupied states from empty ones in the noninteracting fermion case, or a novel Bose surface, where gapless bosonic excitations are anchored. The presence of such surfaces gives rise to logarithmic violation of entanglement entropy area law. When they are convex, we show that the shape of these critical surfaces can be determined by inspecting the leading logarithmic term of real-space entanglement entropy. The fundamental difference between a Fermi surface and a Bose surface is revealed by the fact that the logarithmic terms in entanglement entropies differ by a factor of 2: $S_{\log }^{\mathrm{Bose}}=2S_{\log }^{\mathrm{Fermi}}$, even when they have identical geometry. Our method has remarkable similarity with determining Fermi surface shape using quantum oscillation. We also discuss possible probes of concave critical surfaces in momentum space.

The separation of the Weyl nodes in a broken time-reversal symmetric Weyl semimetal leads to helical quasiparticle excitations at the Weyl nodes which, when coupled with overall spin conservation, allows only internodal transport at the junction of the Weyl semimetal with a superconductor. This leads to an unusual periodic oscillation in the Josephson current as a function of $k_{0}L$, where $L$ is the length of the Weyl semimetal and $2k_0$ is the internodal distance. This oscillation is robust and should be experimentally measurable, providing a direct path to confirming the existence of chiral nodes in the Weyl semimetal.

We introduce an always-on, exchange-only qubit made up of three localized semiconductor spins that offers a true “sweet spot” to fluctuations of the quantum dot energy levels. Both single- and two-qubit gate operations can be performed using only exchange pulses while maintaining this sweet spot. We show how to interconvert this qubit to other three-spin encoded qubits as a resource for quantum computation and communication.