We have observed a superlinear increase of photoluminescence (PL) intensity in a narrow range of excitation intensities for Zn-doped GaN. The characteristic intensity at which the abrupt increase occurs increases with increasing temperature. This is unlike the usual observations for defects in semiconductors in which the PL intensity increases linearly with excitation intensity, saturating at high intensity because defects become saturated with photogenerated charge carriers. The observed phenomenon is attributed to a redirection of electron and hole flow from nonradiative centers at low excitation intensity to a recombination path via the Zn${}_{\text{Ga}}$ acceptor at high excitation intensity. This is the same explanation responsible for the abrupt thermal quenching of PL reported earlier [Reshchikov et al., Phys. Rev. B 84, 075212 (2011).]

Fast quantum oscillations of a charge qubit in a double quantum dot fabricated in a Si/SiGe heterostructure are demonstrated and characterized experimentally. The measured inhomogeneous dephasing time $T_2^{*}$ ranges from 127 ps to 2.1 ns; it depends substantially on how the energy difference of the two qubit states varies with external voltages, consistent with a decoherence process that is dominated by detuning noise (charge noise that changes the asymmetry of the qubit's double-well potential). In the regime with the shortest $T_2^{*}$, applying a charge-echo pulse sequence increases the measured inhomogeneous decoherence time from 127 to 760 ps, demonstrating that low-frequency noise processes are an important dephasing mechanism.

Semiconductor nanowires have an excellent ability to trap, guide, scatter, or absorb light for specific morphology-dependent resonant optical modes. The electromagnetic field enhancement associated with these modes could be used to modify the luminescence of emitters positioned in the vicinity of the nanowire, in a way similar to plasmonic nanostructures. We show that the photoluminescence of a single plane of silicon nanocrystals in silica, positioned at about 3 nm below the surface, can be enhanced by a factor of 2 to 3 in the presence of a silicon nanowire antenna on the silica surface. This could be the basis of a promising fully complementary metal oxide semiconductor compatible process to improve silicon-based light emitting devices, despite a lower enhancement compared to plasmonic nanostructures. Two-dimensional photoluminescence maps recorded for different polarization configurations (incident electric field parallel or perpendicular to the nanowire axis) exhibit different behaviors and can be related to the electric field intensity distribution in the near-field region of the nanowire, where the active silicon nanocrystal layer is located.

Tunneling between opposite surfaces of a topological insulator thin film populated by electrons and holes is considered. We predict the considerable enhancement of tunneling conductivity by Cooper electron-hole pair fluctuations that are a precursor of their Cooper pairing. Cooper pair fluctuations lead to the critical behavior of tunneling conductivity in the vicinity of critical temperature with the critical index $\nu =2$. If the pairing is suppressed by disorder, the behavior of tunneling conductivity in the vicinity of the quantum phase transition is also critical with the index $\mu =2$. The effect can be interpreted as the fluctuational internal Josephson effect, and it is a general phenomenon for electron-hole bilayers. The peculiarities of the effect in other realizations of an electron-hole bilayer are discussed.

We have used low-energy electron microscopy and scanning tunneling microscopy to visualize the dynamics of the formation of one-dimensional Au-induced nanowires on a Ge(001) substrate. At low temperature, the growth of nanowire domains is limited by the diffusion of Au. A wetting-dewetting transition occurs at a temperature of 665 K that transforms nanowire domains into three-dimensional Au clusters. Dewetting occurs at temperatures above 665 K and is fully reversible during repetitive heating and cooling cycles. The decay and growth rates of the nanowire domains below 665 K show intriguing dynamics, caused by the complex diffusion of gold. It correlates directly with the diffracted intensity of a reconstruction along the top of the nanowires, indicating a temperature-dependent potential landscape for the thermally activated diffusion of Au atoms across nanowire domains.

Energy barriers of the key annealing reactions of neutral and charged point defects in SiC are calculated with ab initio density functional theory methods. In order to effectively search for the lowest energy migration paths the preliminary path is first established based on ab initio molecular dynamics (AIMD) simulations. The energy barrier of each hop is then calculated via climbing image nudged elastic band methods for paths guided by the AIMD simulations. The final paths and barriers are determined by comparing different pathways. The annealing reactions have important implications in understanding the amorphization, recovery, and other aspects of the radiation response of SiC. The results are compared with the literature data and experimental results on SiC recovery and amorphization. We propose that the C interstitial and Si antisite annealing reaction may provide a critical barrier that governs both the recovery stage III and amorphization processes.

Disordered systems have grown in importance in the past decades, with similar phenomena manifesting themselves in many different physical systems. Because of the difficulty of the topic, theoretical progress has mostly emerged from numerical studies or analytical approximations. Here, we provide an exact, analytical solution to the problem of uniform phase disorder in a system of identical scatterers arranged with varying separations along a line. Relying on a relationship with Legendre functions, we demonstrate a simple approach to computing statistics of the transmission probability (or the conductance, in the language of electronic transport) and its reciprocal (or the resistance). Our formalism also gives the probability distribution of the conductance, which reveals features missing from previous approaches to the problem.

The conventional Stoner-Wohlfarth astroid describes magnetic switching caused by an external magnetic field in a uniaxial, spatially uniform magnet. In spin-transfer devices the shape of the astroid is modified by the current-induced spin torque. The astroid shape modification is calculated here for a setup with the spin polarizer directed along the easy axis of the magnet, in which case the overall axial symmetry of the device is maintained. Our method does not rely on the assumption of small displacement of equilibria by the spin torque and takes into account that the destabilization of equilibrium states can be caused by two mechanisms: equilibrium merging, already active in a conventional astroid, and local equilibrium destabilization produced by the spin torque. The modified astroid has a self-crossing boundary and demonstrates a region with three stable equilibria. It is shown that the region of stable magnetic energy maximum can be reached only through a narrow bottleneck in the applied field space, which sets some stringent requirements for magnetic field alignment in the experiments. Switching diagrams are then calculated for the setups with magnetic field not perfectly aligned with the easy axis. Features not found in the approximate calculations are revealed and suggestions for their experimental observation are provided.

Scanning tunneling microscopy (STM) can be used to detect inelastic spin transitions in magnetic nanostructures comprising only a handful of atoms. Here we demonstrate theoretically that STM can uniquely identify the electrostatic spin-crossover effect, whereby the exchange interaction between two magnetic centers in a magnetic molecule changes sign as a function of an external electric field. The fingerprint of such effect is a large drop in the differential conductance as the bias increases. Crucially in the case of a magnetic dimer the spin-crossover transition inverts the order between the ground state and the first excited state, but does not change their symmetry. This means that at both sides of the conductance drop associated to the spin-crossover transition there are two inelastic transitions between the same states. The corresponding conductance steps split identically in a magnetic field and provide a unique way to identify the electrostatic spin crossover.

We present detailed temperature- and field-dependent data obtained from magnetization, resistivity, heat capacity, Hall resistivity and thermoelectric power measurements performed on single crystals of CeZn${}_{11}$. The compound orders antiferromagnetically at $\sim$2 K. The zero-field resistivity and thermoelectric power data show features characteristic of a Ce-based intermetallic with crystal-electric-field splitting and possible Kondo lattice effects. We constructed the $T-H$ phase diagram for the magnetic field applied along the easy [110] direction, which shows that the magnetic field required to suppress $T_N$ below 0.4 K is in the range of 45–47.5 kOe. A linear behavior of the $\rho \left(T\right)$ data, $\mathbf{H}\parallel$ [110], was observed only for $H=45$ kOe for 0.46 K $\le T\le$ 1.96 K followed by the Landau–Fermi-liquid regime for a limited range of fields 47.5 kOe $\le H\le$ 60 kOe. From the analysis of our data, it appears that CeZn${}_{11}$ is a local moment compound with little or no electronic correlations arising from the Ce 4$f$ shell. The thermoelectric and transport properties of CeZn${}_{11}$ are mostly governed by the crystal-electric-field effects. Given the very high quality of our single crystals, quantum oscillations are found for both CeZn${}_{11}$ and its nonmagnetic analog LaZn${}_{11}$.

We study the phase diagram of the extended Hubbard model on a two-dimensional square lattice, including on-site ($U$) and nearest-neighbor ($V$) interactions, at weak couplings. We show that the charge-density wave phase that is known to occur at half filling when $4V>U$ gives way to a $d_{xy}$-wave superconducting instability away from half filling, when the Fermi surface is not perfectly nested, and for sufficiently large repulsive $V$ and a range of on-site repulsive interaction $U$. In addition, when nesting is further suppressed and in the presence of a nearest-neighbor attraction, a triplet time-reversal breaking $(p_x+i{p}_y)$-wave pairing instability emerges, competing with the $d_{x^2-y^2}$ pairing state that is known to dominate at fillings just slightly away from half. At even smaller fillings, where the Fermi surface no longer presents any nesting, the $(p_x+i{p}_y)$-wave superconducting phase dominates in the whole regime of on-site repulsions and nearest-neighbor attractions, while $d_{xy}$ pairing occurs in the presence of on-site attraction. Our results suggest that zero-energy Majorana fermions can be realized on a square lattice in the presence of a magnetic field. For a system of cold fermionic atoms on a two-dimensional square optical lattice, both an on-site repulsion and a nearest-neighbor attraction would be required, in addition to rotation of the system to create vortices. We discuss possible ways of experimentally engineering the required interaction terms in a cold atom system.

We investigate the effect of interacting quantum phase slips on persistent current and its fluctuations in ultrathin superconducting nanowires and nanorings pierced by the external magnetic flux. We derive the effective action for these systems and map the original problem onto an effective sine-Gordon theory on torus. We evaluate both the flux-dependent persistent current and the critical radius of the ring beyond which this current gets exponentially suppressed by quantum fluctuations. We also analyze fluctuations of persistent current caused by quantum phase slips. At low temperatures, the supercurrent noise spectrum has the form of coherent peaks which can be tuned by the magnetic flux. Experimental observation of these peaks can directly demonstrate the existence of plasma modes in superconducting nanorings.

Based on the tensor network state representation, we develop a nonlinear dynamic theory, coined network contractor dynamics (NCD), to explore the thermodynamic properties of two-dimensional quantum lattice models. By invoking the rank-1 decomposition in the multilinear algebra, the NCD scheme makes the contraction of the tensor network of the partition function be realized through a contraction of a local tensor cluster with vectors on its boundary. An imaginary-time-sweep algorithm for implementation of the NCD method is proposed for practical numerical simulations. We benchmark the NCD scheme on the square Ising model, which shows great accuracy. Also, the results on the spin-1/2 Heisenberg antiferromagnet on a honeycomb lattice are disclosed to be in good agreement with the quantum Monte Carlo calculations. The quasientanglement entropy $S$, Lyapunov exponent $I^{\text{Lya}}$, and loop character $I^{\text{loop}}$ are introduced within the dynamic scheme, which are found to display “nonlocality” near the critical point, and can be applied to determine the thermodynamic phase transitions of both classical and quantum systems.

Thermal-bias-induced spin angular momentum transfer between a paramagnetic metal and ferromagnetic insulator is studied theoretically based on the stochastic Landau-Lifshitz-Gilbert (LLG) phenomenology. Magnons in the ferromagnet establish a nonequilibrium steady state by equilibrating with phonons via bulk Gilbert damping and electrons in the paramagnet via spin pumping, according to the fluctuation-dissipation theorem. Subthermal magnons and the associated spin currents are treated classically, while the appropriate quantum crossover is imposed on high-frequency magnetic fluctuations. We identify several length scales in the ferromagnet, which govern qualitative changes in the dependence of the thermally induced spin current on the magnetic film thickness.

By measuring electrical Hall, thermal Hall, and Nernst effects, we have comprehensively investigated the topological transport properties associated with the Berry phase of noncoplanar spin structure in a possible skyrmion-lattice phase of MnGe. While the topological terms in thermal and electrical Hall conductivity decrease rapidly with temperature elevation, that in the Nernst effect can be discerned up to near the magnetic transition temperature. The topological term in transverse Peltier conductivity obeys the Mott relation and changes with temperature in proportion to the three-dimensional density of the topological spin texture. This result indicates that three dimensionally ordered skyrmions are responsible for the topological transport phenomena in MnGe.

The magnetothermoelectric characteristics of permalloy film have been investigated by exquisite manipulation of heat flow. Through the angle and field dependence of thermal voltages $V_x$ and $V_y$, the hybrid of the anomalous and the planar Nernst effect can be observed in the combined longitudinal and transverse temperature field. The interplay between the magnetization of Py and the temperature gradient gives rise to the complex field-dependent behaviors of the anomalous and the planar Nernst effect during magnetization reversal.

We study the influence of the band structure on the symmetry and superconducting transition temperature in the (solvable) weak-coupling limit of the repulsive Hubbard model. Among other results we find that (1) as a function of increasing nematicity, starting from the square-lattice (zero nematicity) limit where a nodal $d$-wave state is strongly preferred, there is a smooth evolution to the quasi-1D limit, where a striking near-degeneracy is found between a $p$-wave- and a $d$-wave-type paired states with accidental nodes on the quasi-one-dimensional Fermi surfaces—a situation that may be relevant to the Bechgaard salts. (2) In a bilayer system, we find a phase transition as a function of increasing bilayer coupling from a $d$-wave to an $s_{\pm }$-wave state reminiscent of the iron-based superconductors. (3) When an antinodal gap is produced by charge-density-wave order, not only is the pairing scale reduced, but the symmetry of the pairs switches from $d_{x^2-y^2}$ to $d_{xy}$; in the context of the cuprates, this suggests that were the pseudogap entirely due to a competing CDW order, this would likely cause a corresponding symmetry change of the superconducting order (which is not seen in experiment).

Using Bogoliubov-de Gennes equations we numerically calculate the disorder averaged density of states of disordered semiconductor nanowires driven into a putative topological $p$-wave superconducting phase by spin-orbit coupling, Zeeman spin splitting, and the $s$-wave superconducting proximity effect induced by a nearby superconductor. Comparing with the corresponding theoretical self-consistent Born approximation (SCBA) results treating disorder effects, we comment on the topological phase diagram of the system in the presence of increasing disorder. Although disorder strongly suppresses the zero-bias peak (ZBP) associated with the Majorana zero mode, we find some clear remnant of a ZBP even when the topological gap has essentially vanished in the SCBA theory because of disorder. We explicitly compare effects of disorder on the numerical density of states in the topological and trivial phases.

We introduce a class of superconductors (SCs) in two spatial dimensions with time-reversal symmetry and reflection (i.e., mirror) symmetry. In the absence of interactions, topological classes of these SCs are distinguished by an integer-valued ($\mathbb{Z}$) topological invariant. When interactions are included, we show that the topological classification is modified to ${\mathbb{Z}}_8$. This clearly demonstrates that interactions can have a qualitative effect on topological classifications of gapped states of matter in more than one dimension.

We report on the superconducting properties of a series of heavily doped Si:B epilayers grown by gas immersion laser doping with boron content ($n_B$) ranging from $\sim 3\times {10}^{20}$ cm${}^{-3}$ to $\sim 6\times {10}^{21}$cm${}^{-3}$ and thickness ($d$) varying between $\sim 20$ nm and $\sim 210$ nm. We show that superconductivity is only observed for $n_B$ values exceeding a threshold value ($n_{c,S}$) which scales as $n_{c,S}\propto 1/d$. The critical temperature ($T_c$) then rapidly increases with $n_B$, largely exceeding the theoretical values which can be estimated by introducing the electron-phonon coupling constant (${\lambda }_{e-ph}$) deduced from ab initio calculations into the McMillan equation. Surprisingly $T_c(n_B,d)$ is fully determined by the boron dose ($n_B\times d$) and can be well approximated by a simple $T_c(n_B,d)\approx T_{c,0}[1-A/(n_B.d)]$ law, with $T_{c,0}\sim 750$ mK and $A\sim 8(\pm 1)\times {10}^{15}$ cm${}^{-2}$.