Electron spin dephasing in a singly charged semiconductor quantum dot can partially be suppressed by periodic laser pulsing. We propose a semiclassical approach describing the decoherence of the electron spin polarization governed by the hyperfine interaction with the nuclear spins as well as the probabilistic nature of the photon absorption. We use the steady-state Floquet condition to analytically derive two subclasses of resonance conditions excellently predicting the peak locations in the part of the Overhauser field distribution which is projected in the direction of the external magnetic field. As a consequence of the periodic pulsing, a nonequilibrium distribution develops as a function of time. The numerical simulation of the coupled dynamics reveals the influence of the hyperfine coupling constant distribution onto the evolution of the electron spin polarization before the next laser pulse. Experimental indications are provided for both subclasses of resonance conditions.

We present high-energy x-ray diffraction studies on the structural phases of an optimal high-$T_c$ superconductor ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_{4+y}$ tailored by co-hole-doping. This is specifically done by varying the content of two very different chemical species, Sr and O, respectively, in order to study the influence of each. A superstructure known as staging is observed in all samples, with the staging number $n$ increasing for higher Sr dopings $x$. We find that the staging phases emerge abruptly with temperature, and can be described as a second-order phase transition with transition temperatures slightly depending on the Sr doping. The Sr appears to correlate the interstitial oxygen in a way that stabilizes the reproducibility of the staging phase both in terms of staging period and volume fraction in a specific sample. The structural details as investigated in this paper appear to have no direct bearing on the electronic phase separation previously observed in the same samples. This provides evidence that the electronic phase separation is determined by the overall hole concentration rather than specific Sr/O content and concomitant structural details.

Here we report small-angle neutron scattering (SANS) measurements and theoretical modeling of ${\mathrm{U}}_3{\mathrm{Al}}_2{\mathrm{Ge}}_3$. Analysis of the SANS data reveals a phase transition to sinusoidally modulated magnetic order at $T_{\mathrm{N}}=63$ K to be second order and a first-order phase transition to ferromagnetic order at $T_{\mathrm{c}}=48$ K. Within the sinusoidally modulated magnetic phase $\left(T_{\mathrm{c}}<T<T_{\mathrm{N}}\right)$, we uncover a dramatic change, by a factor of 3, in the ordering wave vector as a function of temperature. These observations all indicate that ${\mathrm{U}}_3{\mathrm{Al}}_2{\mathrm{Ge}}_3$ is a close realization of the three-dimensional axial next-nearest-neighbor Ising model, a prototypical framework for describing commensurate to incommensurate phase transitions in frustrated magnets.

Quantum Monte Carlo simulations offer an unbiased means to study the static and dynamic properties of quantum critical systems, while quantum field theory provides direct analytical results. We study three-dimensional, critical quantum antiferromagnets by performing a combined analysis using both quantum field theory calculations and quantum Monte Carlo data. Explicitly, we analyze the order parameter (staggered magnetization), Néel temperature, quasiparticle gaps, and the susceptibilities in the scalar and vector channels. We connect the two approaches by deriving descriptions of the quantum Monte Carlo observables in terms of the quasiparticle excitations of the field theory. The remarkable agreement not only unifies the description of the static and dynamic properties of the system but also constitutes a thorough test of perturbative O(3) quantum field theory and opens new avenues for the analytical guidance of detailed numerical studies.

Using large-scale simulations based on matrix product state and quantum Monte Carlo techniques, we study the superfluid to Bose glass transition for one-dimensional attractive hard-core bosons at zero temperature, across the full regime from weak to strong disorder. As a function of interaction and disorder strength, we identify a Berezinskii-Kosterlitz-Thouless critical line with two different regimes. At small attraction where critical disorder is weak compared to the bandwidth, the critical Luttinger parameter $K_c$ takes its universal Giamarchi-Schulz value $K_c=3/2$. Conversely, a nonuniversal $K_c>3/2$ emerges for stronger attraction where weak-link physics is relevant. In this strong-disorder regime, the transition is characterized by self-similar power-law-distributed weak links with a continuously varying characteristic exponent $\alpha$.

First-principles molecular dynamics (FPMD) simulations are performed on 6 and 12% Na in dense liquid N. A detailed description of structural and electronic properties leading to an understanding of the effect of Na doping on the polymerization phase transition of N is presented. Compression of the mixtures from 5 to 90 GPa shows three distinct regions of characteristic local order separated by pressures near 30 and 65 GPa. Computation of Gibbs free energies of mixing shows that these mixtures are thermodynamically stable beyond 20 and 15 GPa for 6 and 12% Na, respectively.

Discovery of new materials and improved experimental as well as numerical techniques have led to a renewed interest in geometrically frustrated spin systems. However, there are very few exact results available that can provide a benchmark for comparison. In this work, we calculate exactly the perpendicular susceptibility ${\chi }_{\perp }$ for an Ising antiferromagnet with (i) nearest-neighbor pair interaction on a kagome lattice where strong frustration prevents long-range ordering and (ii) elementary triplet interactions on a kagome lattice which has no frustration but the system remains disordered down to zero temperature. By comparing with other known exact results with and without frustration, we propose that an appropriately temperature-scaled ${\chi }_{\perp }$ can be used as a quantitative measure of the degree of frustration in Ising spin systems.

The exchange interaction between magnetic ions and charge carriers in semiconductors is considered to be a prime tool for spin control. Here, we solve a long-standing problem by uniquely determining the magnitude of the long-range $p-d$ exchange interaction in a ferromagnet-semiconductor (FM-SC) hybrid structure where a 10-nm-thick CdTe quantum well is separated from the FM Co layer by a CdMgTe barrier with a thickness on the order of 10 nm. The exchange interaction is manifested by the spin splitting of acceptor bound holes in the effective magnetic field induced by the FM. The exchange splitting is directly evaluated using spin-flip Raman scattering by analyzing the dependence of the Stokes shift ${\mathrm{\Delta }}_S$ on the external magnetic field $B$. We show that in a strong magnetic field, ${\mathrm{\Delta }}_S$ is a linear function of $B$ with an offset of ${\mathrm{\Delta }}_{pd}=50–100\mu \mathrm{eV}$ at zero field from the FM induced effective exchange field. On the other hand, the $s-d$ exchange interaction between conduction band electrons and FM, as well as the $p-d$ contribution for free valence band holes, are negligible. The results are well described by the model of indirect exchange interaction between acceptor bound holes in the CdTe quantum well and the FM layer mediated by elliptically polarized phonons in the hybrid structure.

Dirac semimetals are three-dimensional analogs of graphene with massless Dirac fermions as low-energy electronic excitations. In contrast to Weyl semimetals, the point nodes in the bulk spectrum of topological Dirac semimetals have a vanishing Chern number, but can yet be stable due to the existence of crystalline symmetries such as uniaxial (discrete) rotation symmetry. We consider a model low-energy Hamiltonian appropriate for the recently discovered topological Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$, and calculate the Nernst response within semiclassical Boltzmann dynamics in the relaxation-time approximation. We show that, for small chemical potentials near the Dirac points, the low-temperature low-magnetic-field Nernst response is dominated by anomalous Nernst effect, arising from a nontrivial profile of Berry curvature on the Fermi surface. Although the Nernst coefficient (both anomalous as well as conventional) vanishes in the limit of zero magnetic field, the low-temperature low-magnetic-field Nernst response, which has an almost steplike profile near $\mathbf{B}=0$, serves as an effective experimental probe of anomalous Nernst effect in topological Dirac semimetals protected by crystalline symmetries. Additionally, we also calculate the Nernst response for a lattice model of an inversion-asymmetric Weyl semimetal for which, in contrast to the case of the Dirac semimetal, we find that the conventional Nernst response dominates over the anomalous. Our calculations in this paper on Nernst response of Dirac semimetals and inversion broken Weyl semimetals are directly relevant to recent experiments on ${\mathrm{Cd}}_3{\mathrm{As}}_2$ (Dirac semimetal) and NbP (inversion broken Weyl semimetal), respectively.

The electronic structures of Prussian blue analog $\left({\mathrm{Rb}}_x{\mathrm{Ba}}_y\right){\mathrm{Mn}}_{[3-(x+2y\left)\right]/2}\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]$ cyanides have been investigated by employing soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) at the Fe and Mn $L$ $\left(2p\right)$ edges. The measured XAS spectra have been analyzed with the configuration-interaction (CI) cluster model calculations. The valence states of the Fe and Mn ions are found to be ${\mathrm{Fe}}^{2+}\text{−}{\mathrm{Fe}}^{3+}$ mixed valent, with an average valency of $v\left(\mathrm{Fe}\right)\sim 2.8$ and nearly divalent $\left({\mathrm{Mn}}^{2+}\right)$, respectively. Our Mn/Fe $2p$ XMCD study supports that ${\mathrm{Mn}}^{2+}$ ions are in the high-spin states while ${\mathrm{Fe}}^{2+}\text{−}{\mathrm{Fe}}^{3+}$ ions are in the low-spin states. The Fe and Mn $2p$ XAS spectra are found to be essentially the same for $80\le T\le 300$ K, suggesting that a simple charge transfer upon cooling from ${\mathrm{Fe}}^{3+}\text{−}\mathrm{CN}\text{−}{\mathrm{Mn}}^{2+}$ to ${\mathrm{Fe}}^{2+}\text{−}\mathrm{CN}\text{−}{\mathrm{Mn}}^{3+}$ does not occur in $\left({\mathrm{Rb}}_x{\mathrm{Ba}}_y\right){\mathrm{Mn}}_{[3-(x+2y\left)\right]/2}\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]$. According to the CI cluster model analysis, it is necessary to take into account both the ligand-to-metal charge transfer and the metal-to-ligand charge transfer in describing Fe $2p$ XAS, while the effect of charge transfer is negligible in describing Mn $2p$ XAS. The CI cluster model analysis also shows that the trivalent ${\mathrm{Fe}}^{3+}$ ions have a strong covalent bonding with the $\mathrm{C}\equiv \mathrm{N}$ ligands and are under a large crystal-field energy of $10Dq\sim 3$ eV, in contrast to the weak covalency effect and a small $10Dq\sim 0.6$ eV for the divalent ${\mathrm{Mn}}^{2+}$ ions.

We critically review the results of magnetic torque measurements on ${\mathrm{SmB}}_6$ that show quantum oscillations. Similar studies have been given two different interpretations. One interpretation is based on the existence of metallic surface states, while the second interpretation is in terms of a three-dimensional Fermi surface involving neutral fermionic excitations. We suggest that the low-field oscillations that are seen by both groups for $B$ fields as small as 6 T might be due to metallic surface states. The high-field three-dimensional oscillations are only seen by one group for fields $B>18$ T. The phenomenon of magnetic breakthrough occurs at high fields and involves the formation of Landau orbits that produces a directional-dependent suppression of Bragg scattering. We argue that the measurements performed under higher-field conditions are fully consistent with expectations based on a three-dimensional semiconducting state with magnetic breakthrough.

In the development of organic electronics, unlimited design possibilities of conjugated polymers offer a wide variety of mechanical and electronic properties. Thereby, it is crucially important to reveal universal physical characteristics that allow efficient and forward developments of new chemical compounds. In particular for organic solar cells, a deeper understanding of exciton dynamics in polymer films can help to improve the charge generation process further. For this purpose, poly(3-hexylthiophene) (P3HT) is commonly used as a model system, although exciton decay kinetics have found different interpretations. Using temperature-dependent time-resolved photoluminescence spectroscopy in combination with low-temperature spectroscopic ellipsometry, we can show that P3HT is indeed a model system in which excitons follow a simple diffusion/hopping model. Based on our results we can exclude the relevance of hot-exciton emission as well as a dynamic torsional relaxation upon photoexcitation on a ps time scale. Instead, we depict the glass transition temperature of polymers to strongly affect exciton dynamics.

We present a theoretical study of spin-velocity injection (SVI) of a pure spin current (PSC) induced by linearly polarized light that impinges normally on the surface of two 50% hydrogenated noncentrosymmetric two-dimensional (2D) graphene structures. The first structure, labeled Up and also known as graphone, is hydrogenated only on one side, and the second, labeled Alt, is 25% hydrogenated at both sides. The hydrogenation opens an energy gap on both structures. The PSC formalism has been developed in the length gauge perturbing Hamiltonian, and includes, through the single-particle density matrix, the excited coherent superposition of the spin-split conduction bands inherent to the noncentrosymmetric nature of the structures considered in this work. We analyze two possibilities: in the first, the spin is fixed along a chosen direction, and the resulting SVI is calculated; in the second, we choose the SVI direction along the surface plane, and calculate the resulting spin orientation. This is done by changing the energy $\hslash \omega$ and polarization angle $\alpha$ of the incoming light. The results are calculated within a full electronic band structure scheme using the density functional theory (DFT) in the local density approximation (LDA). The maxima of the spin velocities are reached when $\hslash \omega =0.084$ eV and $\alpha ={35}^{\circ }$ for the Up structure, and $\hslash \omega =0.720$ eV and $\alpha ={150}^{\circ }$ for the Alt geometry. We find a speed of 668 and 645 km/s for the Up and the Alt structures, respectively, when the spin points perpendicularly to the surface. Also, the response is maximized by fixing the spin-velocity direction along a high-symmetry axis, obtaining a speed of 688 km/s with the spin pointing at ${13}^{\circ }$ from the surface normal, for the Up, and 906 km/s and the spin pointing at ${60}^{\circ }$ from the surface normal, for the Alt system. These speed values are orders of magnitude larger than those of bulk semiconductors, such as CdSe and GaAs, thus making the hydrogenated graphene structures excellent candidates for spintronics applications.

We investigate the influence of the temperature (between 4 K and room temperature) on the fluorescence of colloidal thick-shell CdSe/CdS nanocrystals coupled with a flat gold film. First, we deduce the complex dielectric function $\epsilon$ from the spectrophotometric measurements on the flat gold film. Then, when compared to a nanocrystal deposited on a glass coverslip, calculations show that the enhancement of the photoluminescence decay rate is divided by a factor of 2.3 between 11 and 293 K due to the reduction of optical losses. Our analysis well accounts for the experiments performed on single CdSe/CdS nanocrystals.

The knowledge of lattice thermal conductivity of materials under realistic conditions is vitally important since many modern technologies require either high or low thermal conductivity. Here, we propose a theoretical model for determining lattice thermal conductivity, which takes into account the effect of microstructure. It is based on ab initio description that includes the temperature dependence of the interatomic force constants and treats anharmonic lattice vibrations. We choose ScN as a model system, comparing the computational predictions to the experimental data by time-domain thermoreflectance. Our experimental results show a trend of reduction in lattice thermal conductivity with decreasing domain size predicted by the theoretical model. These results suggest a possibility to control thermal conductivity by microstructural tailoring and provide a predictive tool for the effect of the microstructure on the lattice thermal conductivity of materials based on ab initio calculations.

Andreev bound states (ABSs) are well-defined many-body quantum states that emerge from the hybridization of individual quantum dot (QD) states with a superconductor and exhibit very rich and fundamental phenomena. We demonstrate several electron transport phenomena mediated by ABSs that form on three-terminal carbon nanotube (CNT) QDs, with one superconducting (S) contact in the center and two adjacent normal-metal (N) contacts. Three-terminal spectroscopy allows us to identify the coupling to the N contacts as the origin of the Andreev resonance (AR) linewidths and to determine the critical coupling strengths to S, for which a ground state (or quantum phase) transition in such S-QD systems can occur. In addition, we ascribe replicas of the lowest-energy ABS resonance to transitions between the ABS and odd-parity excited QD states, a process we call excited state ABS resonances. In the conductance between the two N contacts we find a characteristic pattern of positive and negative differential subgap conductance, which we explain by considering two nonlocal processes, the creation of Cooper pairs in S by electrons from both N terminals, and a transport mechanism we call resonant ABS tunneling, possible only in multiterminal QD devices. In the latter process, electrons are transferred via the ABS without effectively creating Cooper pairs in S. The three-terminal geometry also allows spectroscopy experiments with different boundary conditions, for example by leaving S floating. Surprisingly, we find that, depending on the boundary conditions and the device parameters, the experiments either show single-particle Coulomb blockade resonances, ABS characteristics, or both in the same measurements, seemingly contradicting the notion of ABSs replacing the single-particle states as eigenstates of the QD. We qualitatively explain these results as originating from the finite time scale required for the coherent oscillations between the superposition states after a single-electron tunneling event. These experiments demonstrate that three-terminal experiments on a single complex quantum object can also be useful to investigate charge dynamics otherwise not accessible due to the very high frequencies.

We consider the dynamics of an electron in an infinite disordered metallic wire. We derive exact expressions for the probability of diffusive return to the starting point in a given time. The result is valid for wires with or without time-reversal symmetry and allows for the possibility of topologically protected conducting channels. In the absence of protected channels, Anderson localization leads to a nonzero limiting value of the return probability at long times, which is approached as a negative power of time with an exponent depending on the symmetry class. When topologically protected channels are present (in a wire of either unitary or symplectic symmetry), the probability of return decays to zero at long time as a power law whose exponent depends on the number of protected channels. Technically, we describe the electron dynamics by the one-dimensional supersymmetric nonlinear sigma model. We derive an exact identity that relates any local dynamical correlation function in a disordered wire of unitary, orthogonal, or symplectic symmetry to a certain expectation value in the random matrix ensemble of class AIII, CI, or DIII, respectively. The established exact mapping from a one- to a zero-dimensional sigma model is very general and can be used to compute any local observable in a disordered wire.

${}^{51}\mathrm{V}$ NMR measurements have been conducted on a single crystal of the quasi-one-dimensional conductor $\beta \text{−}{\mathrm{Li}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ which undergoes a metal-insulator (MI) transition at $T_{\mathrm{MI}}\sim 170$ K. In the metallic phase, we obtain ${}^{51}\mathrm{V}$ Knight shift and electric field gradient tensors. From the analysis of the ${}^{51}\mathrm{V}$ Knight shifts, we find that the charge disproportionation appears even in the metallic state and the electronic structure is represented within a model of weakly coupled ladders containing two types of ladders with distinct carrier densities. Based on the ${}^{51}\mathrm{V}$ nuclear spin-lattice relaxation rate, we discuss the spin dynamics within the one-dimensional electron gas model. From the analysis of several nonmagnetic ${\mathrm{V}}^{5+}$ spectra observed in the insulating phase, we propose a possible charge-order pattern which has a superlattice modulation larger than those in other family members of $\beta \text{−}{A}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ $(A=\mathrm{Na}$ and Ag). Finally, we discuss the $A$-ion dependence of the electronic structure, the charge disproportionation, and the charge order in $\beta \text{−}{A}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$.

In this article, we discuss the nontrivial collective charge excitations (plasmons) of the extended square lattice Hubbard model. Using a fully nonperturbative approach, we employ the hybrid Monte Carlo algorithm to simulate the system at half-filling. A modified Backus-Gilbert method is introduced to obtain the spectral functions via numerical analytic continuation. We directly compute the single-particle density of states which demonstrates the formation of Hubbard bands in the strongly correlated phase. The momentum-resolved charge susceptibility also is computed on the basis of the Euclidean charge-density-density correlator. In agreement with previous extended dynamical mean-field theory studies, we find that, at high strength of the electron-electron interaction, the plasmon dispersion develops two branches.