Relaxation, $\langle S_z\left(t\right)\rangle$, of the average spin of a carrier in a course of hops over sites hosting random hyperfine fields is studied theoretically. In low dimensions, $d=1,2$, the decay of average spin with time is nonexponential at all times. The origin of the effect is that for $d=1,2$ a typical random-walk trajectory exhibits numerous self-intersections. Multiple visits of the carrier to the same site accelerates the relaxation since the corresponding partial rotations of spin during these visits add up. Another consequence of self-intersections of the random-walk trajectories is that, in all dimensions, the average, $\langle S_z\left(t\right)\rangle$, becomes sensitive to a weak magnetic field directed along $z$. Our analytical predictions are complemented by the numerical simulations of $\langle S_z\left(t\right)\rangle$. The scenario of acceleration of spin relaxation due to returns applies also to the non-Markovian decoherence of a qubit surrounded by multiple fluctuators.

We consider two quantum dots described by the Anderson-impurity model with one electron per dot. The goal of our work is to study the decay of a maximally entangled state between the two electrons localized in the dots. We prepare the system in a perfect singlet and then tunnel couple one of the dots to leads, which induces nonequilibrium dynamics. We identify two cases: If the leads are subject to a sufficiently large voltage and thus a finite current, then direct-tunneling processes cause decoherence and the entanglement as well as spin correlations decay exponentially fast. At zero voltage or small voltages and beyond the mixed-valence regime, virtual-tunneling processes dominate and lead to a slower loss of coherence. We analyze this problem by studying the real-time dynamics of the spin correlations and the concurrence using two techniques, namely, the time-dependent density matrix renormalization group method and a master-equation method. The results from these two approaches are in excellent agreement in the direct-tunneling regime for the case in which the dot is weakly tunnel coupled to the leads. We present a quantitative analysis of the decay rates of the spin correlations and the concurrence as a function of tunneling rate, interaction strength, and voltage.

We calculate the dielectric response of excited crystalline silicon in electron thermal equilibrium by adiabatic time-dependent density functional theory (TDDFT) to model the response to irradiation by high-intensity laser pulses. The real part of the dielectric function is characterized by the strong negative behavior at low frequencies due to excited electron-hole pairs. The response agrees rather well with the numerical pump-probe calculations which simulate electronic excitations in nonequilibrium phase immediately after the laser pulse irradiation. The thermal response is also compared with the Drude model which includes electron effective mass and collision time as fitting parameters. We find that the extracted effective masses are in the range of 0.22–0.36 and lifetimes are in the range of 1–14 fs depending on the temperature. The short Drude lifetimes show that strong damping is possible in the adiabatic TDDFT, despite the absence of explicit electron-electron collisions.

We studied magnetism and morphology of metastable bcc Co nanostructures on Au(001) by x-ray magnetic circular dichroism in combination with scanning tunneling microscopy and photoelectron emission microscopy. While room-temperature deposition of Co onto Au(001) leads to the formation of bcc Co thin films with pure in-plane magnetization, postannealing of these thin films at 500 K drastically changes the morphology to bcc Co islands embedded in Au. In accordance with this morphological change, we find that an out-of-plane magnetization emerges additionally and the coexistence of in-plane and out-of-plane remanent magnetizations is observed for the islands. The nanostructure-size dependence of magnetic moments suggests that the magnetization easy axis of the island changes from the in-plane to the out-of-plane direction with decreasing nanostructure size. Such a spin reorientation transition is likely due to the increased fractional population of rim atoms generating the out-of-plane magnetization in the smaller islands. The observed out-of-plane remanent magnetization of the smaller islands indicates their large magnetic anisotropy.

We report the creation of an artificial skyrmion crystal, which is configurable reliably at room temperature. The samples are fabricated by embedding lithography-patterned arrays of micron-sized Co disks onto Co/Pt multilayer films that have perpendicular magnetic anisotropy. Kerr microscopy and magnetic force microscopy reveal that the disks are in the vortex state with controllable circulation. Via comparison of measured hysteresis loops and calculated ones, we find that the sample can be configured into either a skyrmion or a non-skyrmion state. The reproducible and stable skyrmion crystal at room temperature opens the door to direct exploration of their unique topological properties, which has deservedly caused a flurry of theoretical activity.

We present a detailed investigation of the hole (3% Re) doping effect on the polycrystalline ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$ sample by magnetization, heat capacity, resistivity, muon spin relaxation ($\mu \mathrm{SR}$), and neutron scattering (both elastic and inelastic) measurements. ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$ is an exceptional cerium compound with an unusually high Néel temperature of 27 K. Here we study the stability of the unusual magnetic order by means of controlled doping, and we uncover further surprising attributes of this phase transition. The heat capacity, resistivity, and $\mu \mathrm{SR}$ measurements reveal an onset of magnetic ordering below 23 K, while a broad peak at 31 K (i.e., above $T_N$) has been observed in the temperature dependent susceptibility, indicating an opening of a spin gap above $T_N$. Our important finding, from the neutron diffraction, is that the compound orders antiferromagnetically with a propagation vector $\mathbf{k}=(1,0,0)$ and the ordered state moment is 0.20(1) ${\mu }_B$ along the $b$ axis. This is in sharp contrast to the undoped compound, which shows AFM ordering at 27 K with the ordered moment of 0.34–0.42 ${\mu }_B$ along the $c$ axis. Similar to ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$ our inelastic neutron scattering study on the Re-doped compound shows a sharp spin gap-type excitation near 8 meV at 5 K, but with slightly reduced intensity compared to the undoped compound. Further the excitation broadens and shifts to lower energy ($\le 4$ meV) near 35 K. These results suggest that the low temperature magnetic properties of the hole-doped sample are governed by the competition between the anisotropic hybridization effect and the crystal field anisotropy as observed in hole-doped ${\mathrm{CeOs}}_2{\mathrm{Al}}_{10}$.

We report the discovery of extraordinarily large anisotropy fields and strain-tunable magnetocrystalline anisotropy in ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ epitaxial films. We determine the strain-induced tetragonal distortions and octahedral rotations in ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ epitaxial films grown on (${\mathrm{LaAlO}}_3$)${}_{0.3}$(${\mathrm{Sr}}_2{\mathrm{AlTaO}}_6$)${}_{0.7}$ (LSAT), ${\mathrm{SrTiO}}_3$ (STO), and ${\mathrm{SrCr}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_3$/LSAT substrates using x-ray diffraction and density functional theory. The structural distortions drive dramatic changes in magnetocrystalline anisotropy. We use magnetometry measurements and first principles calculations to determine the atomic origins of the large anisotropy observed. These techniques elucidate the interplay between structural deformations and magnetic behavior and lay the groundwork for the study of other strongly correlated systems in this class of ferromagnetic oxides.

We report the prediction of two ternary chalcogenides that can potentially be used as $p$-type transparent conductors along with experimental synthesis and initial characterization of these compounds, $\mathrm{C}{\mathrm{s}}_2\mathrm{Z}{\mathrm{n}}_{3}C{h}_4\left(Ch=\mathrm{Se},\mathrm{Te}\right)$. In particular, the structures are predicted based on density functional calculations and confirmed by experiments. Phase diagrams, electronic structure, optical properties, and defect properties of $\mathrm{C}{\mathrm{s}}_2\mathrm{Z}{\mathrm{n}}_3\mathrm{S}{\mathrm{e}}_4$ and $\mathrm{C}{\mathrm{s}}_2\mathrm{Z}{\mathrm{n}}_3\mathrm{T}{\mathrm{e}}_4$ are calculated to assess the viability of these materials as $p$-type transparent conducting materials (TCMs). $\mathrm{C}{\mathrm{s}}_2\mathrm{Z}{\mathrm{n}}_3\mathrm{S}{\mathrm{e}}_4$ and $\mathrm{C}{\mathrm{s}}_2\mathrm{Z}{\mathrm{n}}_3\mathrm{T}{\mathrm{e}}_4$, which are stable under ambient air, display large optical band gaps (calculated to be 3.61 and 2.83 eV, respectively) and have small hole effective masses $\left(0.5\text{–}0.77m_{\mathrm{e}}\right)$ that compare favorably with other proposed $p$-type TCMs. Defect calculations show that undoped $\mathrm{C}{\mathrm{s}}_2\mathrm{Z}{\mathrm{n}}_3\mathrm{S}{\mathrm{e}}_4$ and $\mathrm{C}{\mathrm{s}}_2\mathrm{Z}{\mathrm{n}}_3\mathrm{T}{\mathrm{e}}_4$ are $p$-type materials. However, the free hole concentration may be limited by low-energy native donor defects, e.g., Zn interstitials. Nonequilibrium growth techniques should be useful for suppressing the formation of native donor defects, thereby increasing the hole concentration.

We use Sandia's Z machine and magnetically accelerated flyer plates to shock compress liquid krypton to 850 GPa and compare with results from density-functional theory (DFT) based simulations using the AM05 functional. We also employ quantum Monte Carlo calculations to motivate the choice of AM05. We conclude that the DFT results are sensitive to the quality of the pseudopotential in terms of scattering properties at high energy/temperature. A new Kr projector augmented wave potential was constructed with improved scattering properties which resulted in excellent agreement with the experimental results to 850 GPa and temperatures above 10 eV (110 kK). Finally, we present comparisons of our data from the Z experiments and DFT calculations to current equation of state models of krypton to determine the best model for high energy-density applications.

New thin-film Josephson junctions have recently been tested in which the current injected into one of the junction banks governs Josephson phenomena. One thus can continuously manage the phase distribution at the junction by changing the injected current. A method of calculating the distribution of injected currents is proposed for a half-infinite thin-film strip with source-sink points at arbitrary positions at the film edges. The strip width $W$ is assumed small relative to $\Lambda =2{\lambda }^2/d;\lambda$ is the bulk London penetration depth of the film material and $d$ is the film thickness.

We present an analytical study of diamagnetism and transport in a film with superconducting phase fluctuations, formulated in terms of vortex dynamics within the Debye-Hückel approximation. We find that the diamagnetic and Nernst signals decay strongly with temperature in a manner that is dictated by the vortex core energy. Using the theory to interpret Nernst measurements of underdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ above the critical temperature regime, we obtain a considerably better fit to the data than a fit based on Gaussian order-parameter fluctuations. Our results indicate that the core energy in this system scales roughly with the critical temperature and is significantly smaller than expected from BCS theory. Furthermore, it is necessary to assume that the vortex mobility is much larger than the Bardeen-Stephen value in order to reconcile conductivity measurements with the same vortex picture. Therefore, either the Nernst signal is not due to fluctuating vortices, or vortices in underdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ have highly unconventional properties.

We here illustrate the relation between odd-frequency spin-triplet even-parity (OTE) Cooper pairs and anomalous surface magnetic response in time-reversal-invariant (TRI) spin-triplet superfluids and superconductors. The spin susceptibility generally consists of two contributions: even-frequency odd-parity pair amplitudes and odd-frequency even-parity pair amplitudes. The OTE pair amplitudes are absent in the bulk region, but ubiquitously exist in the surface and interface region as Andreev bound states. We here clarify that additional discrete symmetries, originating from the internal symmetry and point-group symmetry, impose strong constraint on the OTE pair amplitudes emergent in the surface of TRI superfluids and superconductors. As a result of the symmetry constraint, the magnetic response of the OTE pairs yields Ising-like anisotropy. For the topological phase of the ${}^3\mathrm{He}\text{−}\mathrm{B}$ in a restricted geometry, the coupling of the OTE pair amplitudes to an applied field is prohibited by an additional discrete symmetry. Once the discrete symmetry is broken, however, the OTE pairs start to couple to the applied field, which anomalously enhances surface spin susceptibility. Furthermore, we extend this theory to TRI superconductors, where the corresponding discrete symmetry is the mirror reflection symmetry.

We consider $d$-dimensional systems which are localized in the absence of interactions, but whose single-particle localization length diverges near a discrete set of (single-particle) energies, with critical exponent $\nu$. This class includes disordered systems with intrinsic or symmetry protected topological bands, such as disordered integer quantum Hall insulators. We show that such marginally localized systems exhibit anomalous properties intermediate between localized and extended, including vanishing dc conductivity but subdiffusive dynamics, and fractal entanglement (an entanglement entropy with a scaling intermediate between area and volume law). We investigate the stability of marginal localization in the presence of interactions, and argue that arbitrarily weak short-range interactions trigger delocalization for partially filled bands at nonzero energy density if $\nu \ge 1/d$. We use the Harris-Chayes bound $\nu \ge 2/d$ to conclude that marginal localization is generically unstable in the presence of interactions. Our results suggest the impossibility of stabilizing quantized Hall conductance at nonzero energy density.

The anisotropy of the electronic structure of the $\mathit{MAX}$ phase ${\mathrm{Cr}}_2\mathrm{AlC}$ has been investigated by electron-energy-loss spectroscopy (EELS) at the C $K$ edge, and x-ray-absorption spectroscopy (XAS) at the Al $K$, Cr $L_{2,3}$, and Cr $K$ edges. The experimental spectra were interpreted using either a multiple-scattering approach or a full-potential band-structure method. The anisotropy is found to be small around C atoms because of the rather isotropic nature of the octahedral site, and of the averaging of the empty C $p$ states probed by EELS at the C $K$ edge. In turn, a pronounced anisotropy of the charge distribution around Al atoms is evidenced from polarized XAS measurements performed on textured ${\mathrm{Cr}}_2\mathrm{AlC}$ sputtered thin films. From the analysis of the XAS data using the multiple-scattering feff code, it is demonstrated that the probed thin film is constituted of $70\%$ (0001) and $30\%$ $\left(10\overline{1}3\right)$ grains oriented parallel to the film surface. A decomposition of the calculated spectrum in coordination shells allows for the ability to connect XAS fine structures to the ${\mathrm{Cr}}_2\mathrm{AlC}$ structure. Combining high-resolution data with up-to-date multiple-scattering calculations, it is shown that the crystalline orientations of the grains present in a probe of $100\times 100$ $\mu \mathrm{m}{}^2$ can be determined from the Cr $K$ edge. Interestingly, it is also revealed that a static disorder is involved in the studied thin films. These findings highlight that, given the overall agreement between experimental and calculated spectra, the ${\mathrm{Cr}}_2\mathrm{AlC}$ electronic structure is accurately predicted using density functional theory.

${\mathrm{SrTiO}}_3$ (STO) is at the core of recent discoveries of two-dimensional electron gas (2DEG) formation at complex oxide interfaces, with the 2DEG residing on the STO side. Experimental results for ultrathin STO layers inserted in ${\mathrm{GdTiO}}_3$ reveal a transition from metallic to insulating behavior, and suggest a strong interplay between electron-electron interaction and lattice distortions. Using first-principles calculations, we show that the metal-to-insulator transition is a bulk property of STO that emerges at extreme doping levels. We find that doping with $1/4$ electron per Ti atom produces a metallic phase as expected, but that adding $1/2$ electron per Ti results in a charge-ordered Mott-insulating phase. The effects of electron-electron interactions and lattice distortions are disentangled, and the Mott-insulator phase is shown to occur in $\mathrm{STO}/{\mathrm{LaAlO}}_3$ and $\text{STO}/{\mathrm{GdTiO}}_3$ heterostructures with ultrathin STO layers.

The influence of phonons on the formation of the excitonic insulator has hardly been analyzed so far. Recent experiments on ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5,1T\text{−}{\mathrm{TiSe}}_2$, and ${\mathrm{TmSe}}_{0.45}{\mathrm{Te}}_{0.55}$, being candidates for realizing the excitonic-insulator state, suggest, however, that the underlying lattice plays a significant role. Employing the Kadanoff-Baym approach we address this issue theoretically. We show that owing to the electron-phonon coupling a static lattice distortion may arise at the excitonic instability. Most importantly such a distortion will destroy the acoustic phase mode being present if the electron-hole pairing and condensation is exclusively driven by the Coulomb interaction. The absence of off-diagonal long-range order, when lattice degrees of freedom are involved, challenges that excitons in these materials form a superfluid condensate of Bose particles or Cooper pairs composed of electrons and holes.

We study the ratio of the energy and particle currents ($j^E/j^N$) in an integrable one-dimensional system of interacting fermions. Both currents are driven by a finite (nonzero) dc electric field. In doped insulators, where the local conserved quantities saturate the so-called Mazur bound on the charge stiffness, $j^E/j^N$ agrees with the linear-response theory, even though such agreement may be violated for each current alone. However, in the metallic regime with a nonsaturated Mazur bound, the ratio $j^E/j^N$ in a driven system is shown to be much larger than predicted by the linear-response theory.

We investigate the effects of Rashba and intrinsic spin-orbit couplings (SOC) in graphynes. First, we develop a general method to address spin-orbit couplings within the tight-binding theory. Then, we apply this method to $\alpha$-, $\beta$-, and $\gamma$-graphyne, and determine the SOC parameters in terms of the microscopic hopping and onsite energies. We find that for $\alpha$-graphyne, as in graphene, the intrinsic SOC opens a nontrivial gap, whereas the Rashba SOC splits each Dirac cone into four. In $\beta$- and $\gamma$-graphyne, the Rashba SOC can lead to a Lifshitz phase transition, thus transforming the zero-gap semiconductor into a gapped system or vice versa, when pairs of Dirac cones annihilate or emerge. The existence of internal (within the benzene ring) and external SOC in these compounds allows us to explore a myriad of phases not available in graphene.