We report ${}^{63,65}\mathrm{Cu}$-NMR spectroscopy and Knight shift measurements on a single crystal of the electron-doped high-$T_c$ superconductor ${\mathrm{Pr}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4-y}$ with an applied magnetic field $\left(H\right)$ up to 26.42 T. A very small NQR frequency is obtained with the observation of the spectrum, which shows an extremely wide continuous distribution of it that becomes significantly narrower below 20 K at $H\parallel c$ where the superconductivity is completely suppressed, indicating a significant change in the charge distribution at the Cu site, while the corresponding change at $H\perp c$ is negligible when the superconductivity is present or not fully suppressed. The Knight shift and central linewidth are proportional to the applied magnetic field with a high anisotropy. We find that the magnitude of the internal static magnetic field at the copper is dominated by the anisotropic ${\mathrm{Cu}}^{2+}$ $3d$ orbital contributions, while its weak temperature dependence is mainly determined by the isotropic contact hyperfine coupling to the paramagnetic ${\mathrm{Pr}}^{3+}$ spins, which also gives rise to the full distribution of the internal static magnetic field at the copper for $H\perp c$. This internal static electric and magnetic field environment at the copper is very different from that in the hole-doped cuprates, and may provide new insight into the understanding of high-$T_c$ superconductivity. Other experimental techniques are needed to verify whether the observed significant narrowing of the charge distribution at the Cu site with $H\parallel c$ is caused by the charge ordering [E. H. da Silva Neto et al., Science (to be published, 2014)] or a new type of charge modulation.

The cuprate high temperature superconductors exhibit a pronounced trend in which the superconducting transition temperature $T_{\mathrm{c}}$ increases with the number of ${\mathrm{CuO}}_2$ planes $n$ in the crystal structure. We compare the magnetic excitation spectrum of ${\mathrm{Bi}}_{2+x}{\mathrm{Sr}}_{2-x}{\mathrm{CuO}}_{6+\delta }$ (Bi-2201) and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{10+\delta }$ (Bi-2223), with $n=1$ and 3, respectively, using Cu $L_3$-edge resonant inelastic x-ray scattering. Near the antinodal zone boundary we find the paramagnon energy in Bi-2223 is substantially higher than that in Bi-2201, indicating that multilayer cuprates host stronger effective magnetic exchange interactions, providing a possible explanation for the $T_{\mathrm{c}}$ vs $n$ scaling. In contrast, the nodal direction exhibits very strongly damped, almost nondispersive excitations. We argue that this implies that the magnetism in the doped cuprates is partially itinerant in nature.

The jerky character of austenite-martensite phase transformation in ${\mathrm{Ni}}_2\mathrm{MnGa}$ single crystals (with 10M martensite structure) has been investigated by thermal cycling using a differential scanning calorimeter (DSC) and by detection of acoustic emissions (AEs) at low cooling and heating rates (0.1 K/min and below). It is illustrated that, besides the low cooling and heating rate, mass and surface roughness are also important parameters in optimizing the best signal/noise ratio in order to obtain individual peaks suitable for statistical analysis. Three types of samples, differing in the twin structure and twin boundary behavior, were investigated with and without surface roughening made by electro-erosion. The statistical analysis, carried out for both (thermal and acoustic) types of signals, provided power-law behavior. In calorimetric measurements the energy exponents, obtained in cooling, were the same within the experimental errors $\left(\varepsilon =1.7\pm 0.2\right)$ for the three samples investigated. In acoustic emission experiments the energy and amplitude, $\alpha$, exponents were determined both for cooling and heating. The exponents for cooling and heating runs are slightly different. They are larger for heating for both $\alpha$ and $\varepsilon$, in accordance with the asymmetric acoustic activity: we observed higher acoustic activity (higher number of hits) during cooling. The effect of the surface roughness is negligible in the exponents (but higher acoustic activity corresponds to higher roughness) and the following values were obtained: $\varepsilon =1.5\pm 0.1$ and $\alpha =2.1\pm 0.1$ for cooling as well as $\varepsilon =1.8\pm 0.1$ and $\alpha =2.6\pm 0.1$ for heating. Our results are in accordance with the results of Gallardo et al. [Phys. Rev. B 81, 174102 (2010)] obtained in Cu based alloys: the exponents of the energy distributions, for both DSC and AE signals, were the same within the experimental errors. Furthermore, our exponents obtained from the AE measurements are close to the values obtained by Ludwig et al. ($\alpha =2.6\pm 0.1$ and $\varepsilon =1.75\pm 0.1$) [App. Phys. Lett. 94 121901 (2009)] and Niemann et al. ($\varepsilon =1.9\pm 0.1$) [Phys. Rev. B 86, 214101 (2012)] in ${\mathrm{Ni}}_2\mathrm{MnGa}$ alloys with similar 10M martensite structure.

One of the most accepted models that describe the anomalous thermal behavior of amorphous materials at temperatures below 1 K relies on the quantum mechanical tunneling of atoms between two nearly equivalent potential energy wells forming a two-level system (TLS). Indirect evidence for TLSs is widely available. However, the atomistic structure of these TLSs remains an unsolved topic in the physics of amorphous materials. Here, using classical molecular dynamics, we found several hitherto unknown bistable structural motifs that may be key to understanding the anomalous thermal properties of amorphous alumina at low temperatures. We show through free energy profiles that the complex potential energy surface can be reduced to canonical TLSs. The tunnel splitting predicted from instanton theory, the number density, dipole moment, and coupling to external strain of the discovered motifs are consistent with experiments.

The cubic symmetry of pyrochlore iridium oxides $R_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ ($R=\mathrm{Nd}$, Eu, and Pr) has been investigated by high resolution x-ray diffraction experiments down to 4 K, in order to clarify the relationship between the metal-insulator transition (MIT) and the small structural phase transition suggested by Raman scattering experiments in these compounds. We have found that a small negative thermal expansion of the order of ${10}^{-3}$ Å appears only in ${\mathrm{Nd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ below the MIT, $T_{\mathrm{MIT}}=34$ K, ascribable to the magnetovolume effect of the long-range order of Ir moments. However, any breaking of the cubic symmetry of three iridates has not been observed as appearance of superlattice reflections nor splittings of cubic reflections below $T_{\mathrm{MIT}}$. These results imply that lowering of the cubic symmetry plays a minor role for the change in the electronic state of these compounds, while a magnetic order of Ir moments plays a major role for the MIT.

We study ${\mathbb{Z}}_3$ symmetry-protected topological (SPT) phases in one-dimensional spin systems with $Z_3\times Z_3$ symmetry. We construct ground-state wave functions of the matrix product form for nontrivial ${\mathbb{Z}}_3$ phases and their parent Hamiltonian from a cocycle of the group cohomology $H^2(Z_3\times Z_3,U\left(1\right))$. The Hamiltonian is an SU(3) version of the Affleck-Kennedy-Lieb-Tasaki (AKLT) model, consisting of bilinear and biquadratic terms of su(3) generators in the adjoint representation. A generalization to the $\mathrm{SU}\left(N\right)$ case, the $\mathrm{SU}\left(N\right)$ AKLT Hamiltonian, is also presented, which realizes nontrivial ${\mathbb{Z}}_N$ SPT phases. We use the infinite-size variant of the density matrix renormalization group (iDMRG) method to determine the ground-state phase diagram of the SU(3) bilinear-biquadratic model as a function of the parameter $\theta$ controlling the ratio of the bilinear and biquadratic coupling constants. The nontrivial ${\mathbb{Z}}_3$ SPT phase is found for a range of the parameter $\theta$ including the point of vanishing biquadratic term $\left(\theta =0\right)$ as well as the SU(3) AKLT point [$\theta =arctan(2/9)$]. A continuous phase transition to the SU(3) dimer phase takes place at $\theta \approx -0.027\pi$, with a central charge $c\approx 3.2$. For SU(3) symmetric cases, we define string order parameters for the ${\mathbb{Z}}_3$ SPT phases in a similar way to the conventional Haldane phase. We propose simple spin models that effectively realize the SU(3) and SU(4) AKLT models.

We use a combination of dynamical mean-field model calculations and LDA + U material specific calculations to investigate the low temperature phase transition in the compounds from the $\left({\mathrm{Pr}}_{1-y}R{}_y\right){}_x{\mathrm{Ca}}_{1-x}{\mathrm{CoO}}_3$ $\left(R=\text{Nd,}\text{Sm,}\text{Eu,}\text{Gd,}\text{Tb,}\text{Y}\right)$ family (PCCO). The transition, marked by a sharp peak in the specific heat, leads to an exponential increase of dc resistivity and a drop of the magnetic susceptibility, but no order parameter has been identified yet. We show that condensation of spin-triplet, atomic-size excitons provides a consistent explanation of the observed physics. In particular, it explains the exchange splitting on the Pr sites and the simultaneous Pr valence transition. The excitonic condensation in PCCO is an example of a general behavior expected in certain systems in the proximity of a spin-state transition.

A Cooper pair splitter consists of a central superconducting contact, S, from which electrons are injected into two parallel, spatially separated quantum dots (QDs). This geometry as well as electron interactions can lead to correlated electrical currents due to the spatial separation of spin-singlet Cooper pairs from S. We present experiments on such a device with a series of bottom gates, which allows for spatially resolved tuning of the tunnel couplings between the QDs and the electrical contacts and between the two QDs. Our main findings are gate-induced transitions between positive conductance correlation in the QDs due to Cooper pair splitting and negative correlations due to QD dynamics. Using a semiclassical rate equation model we show that the experimental findings are consistent with in situ electrical tuning of the local and nonlocal quantum transport processes. In particular, we illustrate how the competition between Cooper pair splitting and local processes can be optimized in such hybrid nanostructures.

We show theoretically that strong electron coupling to circularly polarized photons in non-singly-connected nanostructures results in the appearance of an artificial gauge field that changes the electron phase. The effect arises from the breaking of time-reversal symmetry and is analogous to the well-known Aharonov-Bohm phase effect. It can manifest itself in the oscillations of conductance as a function of the intensity and frequency of the illumination. The theory of the effect is elaborated for mesoscopic rings in both ballistic and diffusive regimes.

In this manuscript we present an ab initio picture of ultrafast magneto-optical dynamics in clusters containing 2, 3, and 4 Ni atoms. The presence of the magnetic centers in the clusters renders our systems of choice highly interesting for studying ultrafast spin dynamics. Here we systematically study functional cooperativity by increasing both the number of active centers and the spin multiplicities included in our Hilbert space (singlets $\to$ triplets $\to$ quintets), and deriving several ultrafast, laser-driven, spin-manipulation scenarios. Our results indicate various cooperative effects like spin flip by the M process, and simultaneous spin flip and spin transfer, as well as reversible and irreversible demagnetization scenarios. As it turns out the functional cooperativity of the clusters strongly benefits from the delicate interplay of the spin multiplicity and the number of active centers.

We study the Hubbard model on the honeycomb lattice with nearest-neighbor hopping $\left(t>0\right)$ and next-nearest-neighbor hopping $\left(t^{\prime }<0\right)$. When $t^{\prime }<-t/6$, the single-particle spectrum is featured by the continuously distributed Van Hove saddle points at the band bottom, where the density of states diverges in a power law. We investigate possible unconventional superconductivity in such systems with the Fermi level close to the band bottom by employing both random-phase-approximation and determinant quantum Monte Carlo approaches. Our study reveals a possible triplet $p+ip$ superconductivity in this system with appropriate interactions. Our results might provide a possible route to look for triplet superconductivity with relatively high transition temperature in low-filled graphene and other similar systems.

The chiral two-dimensional electron gas in Landau levels $\left|N\right|>0$ of a Bernal-stacked graphene bilayer has a valley-pseudospin Ising quantum Hall ferromagnetic behavior at odd filling factors ${\nu }_N=1,3$ of these fourfold degenerate states. At zero interlayer electrical bias, the ground state at these fillings is spin polarized and electrons occupy one valley or the other while a finite electrical bias produces a series of valley pseudospin-flip transitions. In this work, we discuss the hysteretic behavior of the Ising quantum Hall ferromagnets. We compute the transport gap due to different excitations: bulk electron-hole pairs, electron-hole pairs confined to the coherent region of a valley-pseudospin domain wall, and spin or valley-pseudospin skyrmion-antiskyrmion pairs. We determine which of these excitations has the lowest energy at a given value of the Zeeman coupling, bias, and magnetic field.

We present an ab initio study of the spin-resolved optical conductivity of two-dimensional (2D) group-VIB transition-metal dichalcogenides (TMDs). We carry out fully relativistic density-functional-theory calculations combined with maximally localized Wannier functions to obtain band manifolds at extremely high resolutions and focus on the photoresponse of 2D TMDs to circularly polarized light in a wide frequency range. We present extensive numerical results for monolayer TMDs involving molybdenum and tungsten combined with sulfur and selenium. Our numerical approach allows us to locate with a high degree of accuracy the positions of the points in the Brillouin zone that are responsible for Van Hove singularities in the optical response. Surprisingly, some of the saddle points do not occur exactly along high-symmetry directions in the Brillouin zone, although they happen to be in their close proximity.

The anomalous Hall effect (AHE) in the “dirty region” $\left({\sigma }_{xx}<{10}^4 \text{S}/\text{cm}\right)$ is studied in amorphous ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ (CoFeB) thin films. Ostensively the conventionally adopted scaling $\left({\sigma }_{\text{AH}}\propto {\sigma }_{xx}^{1.6}\right)$ appears to be valid, but we find that the results can be better explained by the proper scaling [${\sigma }_{\text{AH}}=-(\alpha {\sigma }_{xx0}^{-1}+\beta {\sigma }_{xx0}^{-2}){\sigma }_{xx}^2-b$]. This implies that the AHE in the dirty region can also be well described by the standard skew scattering, side jump, and the intrinsic mechanisms.

Neutron spectroscopy measurements reveal short-range spin correlations near and above the ferromagnetic-paramagnetic phase transition in manganite materials of the form $\mathrm{La}{}_{1-x}{A}_x{\mathrm{MnO}}_3$, including samples with an insulating ground state as well as colossal magnetoresistive samples with a metallic ground state. Quasielastic magnetic scattering is revealed that forms clear ridges running along the [100]-type directions in momentum space. A simple model consisting of a conduction electron hopping between spin-polarized Mn ions that becomes self-trapped after a few hops captures the essential physics of this magnetic component of the scattering. We associate this scattering component with the magnetic part of diffuse polarons, as we observe a temperature dependence similar to that of the diffuse structural scattering arising from individual polarons.

We propose an all optical ultrafast method to highly magnetize general quantum magnets using a circularly polarized terahertz laser. The key idea is to utilize a circularly polarized laser and its chirping. Through this method, one can obtain magnetization curves of a broad class of quantum magnets as a function of time even without any static magnetic field. We numerically demonstrate the laser-induced magnetization process in realistic quantum spin models and find a condition for the realization. The onset of magnetization can be described by a many-body version of Landau-Zener mechanism. In a particular model, we show that a plateau state with topological properties can be realized dynamically.

Although Mn impurities are promising to bring Si, the most widespread semiconductor employed in electronic devices, into the spintronics realm, few theoretical works exist that calculate the charge transition levels of Mn in Si. Among these works, none of them makes use of gap correction methods. To fill this void, we performed first principles calculations for Mn-doped Si, using the GGA-1/2 method, which approximately includes quasiparticle corrections at a small computational price. Our results improve the theoretical description of these charge transition levels, achieving good agreement with experimental results for interstitial and substitutional sites. Furthermore, the GGA-1/2 method allowed us to use reasonably large supercells, up to 217 atoms.

Spin-cluster excitations induced by microwave radiation have been detected as a function of an applied magnetic field in the rare-earth kagome system $\mathrm{N}{\mathrm{d}}_3\mathrm{G}{\mathrm{a}}_5\mathrm{Si}{\mathrm{O}}_{14}$. The results are interpreted using a Heisenberg model which allows for anisotropic exchange interactions between neighboring spins. At a microwave frequency of 120 GHz the conventional independent-ion electron spin resonance (ESR) spectrum, due to Zeeman splitting of the ground state Kramers doublet, is not observed at low temperatures. Instead, a large number of resonances are seen that are shifted from the conventional ESR position. These resonances persist to temperatures well above 20 K with similar behavior found at 230 GHz. The observations are consistent with the excitation of spin waves in short-range antiferromagnetically correlated spin loops or clusters and are similar to the ESR spectral features seen previously in the $\mathrm{P}{\mathrm{r}}_3\mathrm{G}{\mathrm{a}}_5\mathrm{Si}{\mathrm{O}}_{14}$ kagome system. Upon increasing the temperature and/or the microwave frequency, a broad Zeeman resonance does eventually emerge at the predicted ground state doublet resonance field.

A chiral time-reversal symmetry breaking $d+id$-wave superconducting state is likely to emerge in graphene doped close to the Van Hove singularity. As heavy doping procedures are expected to introduce defects, we investigate here the effects of microscopic defects on the $d+id$-wave superconducting state at the Van Hove singularity. We find that, while the superconducting order is reduced near a defect, the $d+id$-wave state remains intact and recovers in an exponential manner away from the defect. The recovery length is found to be on the order of one lattice constant for weak couplings, and, as we show, this is comparable to the recovery length of a conventional $s$-wave state on the graphene honeycomb lattice, thereby demonstrating that the unconventional $d+id$-wave state is quite resilient to defects. Moreover, we find no significant changes between a single site defect and more extended defects, such as a bivacancy. While the $d+id$-wave state is fully gapped, we also show that defects introduce localized midgap states with nonzero energies, which should be accessible via scanning probe experiments.

We report the observation of quantum stochastic synchronization (QSS) in a dissipative quantum system—a moderately damped current-biased Josephson junction. We show that when the junction is biased subcritically by a combination of a constant current and a small sinusoidal modulation signal, forced phase synchronization between the otherwise random, incoherent quantum tunneling and the signal may arise as a consequence of the interplay between quantum fluctuation and the weak periodic drive. It is found that optimal synchronization occurs at a signal frequency $f_{\text{QSS}}$ that is comparable to the quantum tunneling rate. We also find a phenomenon called “modulation induced suppression of quantum noise”: for signal frequency well below $f_{\text{QSS}},$ the power spectral density of escape time distribution of the junction could be reduced substantially from the floor level of the unmodulated system. Furthermore, we demonstrate that the application of a small signal with proper phase can suppress the average tunneling rate and enhance the stability of the metastable system. Our experimental results agree well with the numerical calculations without the use of adjustable parameters.