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arXiv2010

Statistical model of charge transport in colloidal quantum dot array

A new statistical model of charge transport in colloidal quantum dot arrays is proposed. It takes into account Coulomb blockade forbidding multiple occupancy of nanocrystals and influence of energetic disorder of interdot space. The model explains power law current transients and the presence of memory effect. The fractional differential analogue of the Ohm law is found phenomenologically for nanocrystal arrays. The model combines ideas that were considered as conflicting by other authors: the Scher-Montroll idea about power law distribution of waiting times in localized states for disordered semiconductors is applied with taking into account Coulomb blockade, Novikov's condition about asymptotical power law distribution of time intervals between successful current pulses in conduction channels is fulfilled, carrier injection blocking predicted by Ginger and Greenham takes place.

cond-mat.dis-nn↗

arXiv2010

Controlling diffusive transport in confined geometries

We analyze the diffusive transport of Brownian particles in narrow channels with periodically varying cross-section. The geometrical confinements lead to entropic barriers, the particle has to overcome in order to proceed in transport direction. The transport characteristics exhibit peculiar behaviors which are in contrast to what is observed for the transport in potentials with purely energetic barriers. By adjusting the geometric parameters of the channel one can effectively tune the transport and diffusion properties. A prominent example is the maximized enhancement of diffusion for particular channel parameters. The understanding of the role of channel-shape provides the possibility for a design of stylized channels wherein the quality of the transport can be efficiently optimized.

cond-mat.stat-mech↗

arXiv2010

Unified quaternionic description of charge and spin transport and intrinsic non-linearity of spin currents

We present a unified theory of charge and spin transport using quaternionic formalism. It is shown that both charge current and spin currents can be combined together to form a quaternionic current. The scalar and vector part of quaternionic currents correspond to charge and spin current respectively. We formulate a unitarity condition on the scattering matrix for quaternionic current conservation. It is shown that in presence of spin flip interactions a weaker quaternionic unitarity condition implying charge flux conservation but spin flux non conservation is valid. Using this unified theory we find that spin currents are intrinsically non linear. Its implication for recent experimental observation of spin generation far away from the boundaries are discussed.

cond-mat.mes-hall↗

arXiv2010

Phase fluctuations in finite thickness disordered superconducting thin films

Phase fluctuations in finite thickness layered superconducting films are studied theoretically. The model consists of a set of layers, coupled to each other via a gradient-like term in the phase-only action. It is shown that the effective phase stiffness of the whole system is proportional to its thickness (i.e. the number of individual layers). This result leads to a superconductor-insulator transition and an inverse dependence of $T_c$ on thickness,$T_c\sim d^{-1}$, both in accord with the experimental findings. The relevance to disordered thin films is discussed.

cond-mat.dis-nn↗

arXiv2011

Non-local effects of edge excitations in the quantum Hall regime

We use a novel sample geometry to study non-local effects of edge excitations in the integer quantum Hall effect regime. We find that the condition of local equilibrium at the quantum Hall edge is affected by the diffusion of dynamically polarized nuclei. Our analysis indicates, that the nuclear diffusion is effectively one-dimensional in the present experiment.

cond-mat.mes-hall↗

arXiv2011

An ultrasensitive spintronic strain sensor

We propose a spintronic strain sensor capable of sensing strain with a sensitivity of 1E-13/sqrt{Hz} at room temperature with an active sensing area of 1 cmE2 and power dissipation of 1 watt. This device measures strain by monitoring the change in the spin-polarized current in a parallel array of free standing nanowire spin valves when the array is subjected to compressive or tensile stress along the wires' length. The change in the current is linearly proportional to the strain, which makes the sensor relatively distortion-free. Such a sensor can be fabricated using a variety of techniques involving nanolithography, self assembly and epitaxial growth.

cond-mat.mes-hall↗

arXiv2010

Resonance Scattering of Waves in Chaotic Systems

This is a brief overview of RMT applications to quantum or wave chaotic resonance scattering, focusing mainly on theoretical results obtained via non-perturbative methods starting from mid-nineties.

cond-mat.mes-hall↗

arXiv2010

Quantum discrete breathers

We review recent studies about quantum discrete breathers. We describe their basic properties in comparison with their classical counterparts, and the ways they may be addressed theoretically in different quantum lattice models including either bosonic or fermionic excitations. We also review recent experimental work in the field.

cond-mat.mes-hall↗

arXiv2012

Robust Surface Doping of Bi$_2$Se$_3$ by Rb Intercalation

Rubidium adsorption on the surface of the topological insulator Bi$_2$Se$_3$ is found to induce a strong downward band bending, leading to the appearance of a quantum-confined two dimensional electron gas states (2DEGs) in the conduction band. The 2DEGs shows a strong Rashba-type spin-orbit splitting, and it has previously been pointed out that this has relevance to nano-scale spintronics devices. The adsorption of Rb atoms, on the other hand, renders the surface very reactive and exposure to oxygen leads to a rapid degrading of the 2DEGs. We show that intercalating the Rb atoms, presumably into the van der Waals gaps in the quintuple layer structure of Bi$_2$Se$_3$, drastically reduces the surface reactivity while not affecting the promising electronic structure. The intercalation process is observed above room temperature and accelerated with increasing initial Rb coverage, an effect that is ascribed to the Coulomb interaction between the charged Rb ions. Coulomb repulsion is also thought to be responsible for a uniform distribution of Rb on the surface.

cond-mat.mtrl-sci↗

arXiv2013

Global Completability with Applications to Self-Consistent Quantum Tomography

Let p_1, ..., p_N \in R^D be unknown vectors and let Omega \subseteq {1,...,N}^{\times 2}. Assume that the inner products p_i^T p_j are fixed for all (i,j) \in Omega. Do these inner product constraints (up to simultaneous rotation of all vectors) determine p_1, ..., p_N uniquely? Here we derive a necessary and sufficient condition for the uniqueness of p_1, ...,p_N (i.e., global completability) which is applicable to a large class of practically relevant sets Omega. Moreover, given Omega, we show that the condition for global completability is universal in the sense that for almost all vectors p_1, ...,p_N \in R^D the completability of p_1, ...,p_N only depends on Omega and not on the specific values of p_i^T p_j$ for (i,j) \in Omega. This work was motivated by practical considerations, namely, self-consistent quantum tomography.

quant-ph↗

arXiv2014

Giant spin torque in systems with anisotropic exchange interaction

Control of magnetic domain wall movement by the spin-polarized current looks promising for creation of a new generation of magnetic memory devices. A necessary condition for this is the domain wall shift by a low-density current. Here I show that a strongly anisotropic exchange interaction between mobile heavy holes and localized magnetic moments enormously increases the current-induced torque on the domain wall as compared to systems with isotropic exchange. This enables one to control the domain wall motion by current density 10^4 A/cm^2 in ferromagnet/semiconductor hybrids. The experimental observation of the anisotropic torque will facilitate the integration of ferromagnetism into semiconductor electronics

cond-mat.mes-hall↗

arXiv2013

Tuning the quantum oscillations of surface Dirac electrons in the topological insulator Bi$_2$Te$_2$Se by liquid gating

In Bi$_2$Te$_2$Se, the period of quantum oscillations arising from surface Dirac fermions can be increased 6-fold using ionic liquid gating. At large gate voltages, the Fermi energy reaches the $N$ = 1 Landau level in a 14-Tesla field. This enables the $\frac12$-shift predicted for the Dirac spectrum to be measured accurately. A surprising result is that liquid gating strongly enhances the surface mobility. By analyzing the Hall conductivity, we show that the enhancement occurs on only one surface. We present evidence that the gating process is fully reversible (hence consistent with band-bending by the $E$-field from the anion layer accumulated). In addition to the surface carriers, the experiment yields the mobility and density of the bulk carriers in the impurity band. By analyzing the charge accumulation vs. gate voltage, we also obtain estimates of the depletion width and the areal depletion capacitance $C_d/A$. The value of $C_d/A$ implies an enhanced electronic polarizability in the depletion region.

cond-mat.mes-hall↗

arXiv2013

High temperature plasmonics: Narrowband, tunable, near-field thermal sources

We propose multiple approaches for controlling Wien's displacement law in the near-field leading to narrowband, tunable, spatially-coherent high temperature thermal sources. We show that narrowband super-planckian thermal emission relevant to the wavelength ranges for near-field thermophotovoltaics can be achieved by utilizing engineered plasmonic states of metals with a high melting point. Along with conventional thin film surface plasmon polaritons of high temperature plasmonic metals, we also introduce multiple different approaches to narrowband near field thermal emission a) epsilon-near-zero metamaterials b) tunable surface-plasmon-polaritons of anisotropic metamaterials and c) photonic van hove singularities. Our work solves the critical issue of a narrowband thermal source for near-field thermophotovoltaics.

physics.optics↗

arXiv2013

A versatile simulator for specular reflectivity study of multi-layer thin films

A versatile X-ray/neutron reflectivity (specular) simulator using LabVIEW(National Instruments Corp.) for structural study of a multi-layer thin film having any combination, including the repetitions, of nano-scale layers of different materials is presented here (available to download from the link provided at the end of the paper). Inclusion of absorption of individual layers, inter-layer roughnesses, background counts, beam width, instrumental resolution and footprint effect due to finite size of the sample makes the simulated reflectivity close to practical one. The effect of multiple reflection is compared with simulated curves following the exact dynamical theory and approximated kinematical theory. The applicability of further approximation (Born theory) that the incident angle does not change significantly from one layer to another due to refraction is also considered. Brief discussion about reflection from liquid surface/interface and reflectivity study using polarized neutron are also included as a part of the review. Auto-correlation function in connection with the data inversion technique is discussed with possible artifacts for phase-loss problem. An experimental specular reflectivity data of multi-layer erbium stearate Langmuir-Blodgett (LB) film is considered to estimate the parameters by simulating the reflectivity curve.

cond-mat.mes-hall↗

arXiv2013

Inverted random nanopyramids patterning for crystalline silicon photovoltaics

We demonstrate a nanopatterning technique for silicon photovoltaics, which optically outperforms conventional micron-scale random pyramids, while decreasing by a factor of ten the quantity of silicon lost during the texturing process. We combine hole-mask colloidal lithography, a bottom-up nanolithography technique, with reactive ion etching to define nanopyramids at the surface of a silicon wafer. Thanks to the self-organised aspect of the technique, the beads are randomly distributed, however keeping a interbead distance of the order of their diameter. We tune the nanopattern feature size to maximize the absorption in the crystalline silicon by exploiting both anti-reflection and light trapping. When optimized, the nanopyramids lead to a higher absorption in the crystalline silicon than the conventional micron-scale random pyramids in the visible and near the band edge, with a superior robustness to variations of the angle of the incident light. As the nanopatterning technique presented here is simple, we expect that it could be readily integrated into the crystalline silicon solar cell fabrication processing.

cond-mat.mes-hall↗

arXiv2013

Electron-phonon coupling-induced kinks in the sigma band of graphene

Angle-resolved photoemission spectroscopy reveals pronounced kinks in the dispersion of the sigma band of graphene. Such kinks are usually caused by the combination of a strong electron-boson interaction and the cut-off in the Fermi-Dirac distribution. They are therefore not expected for the $σ$ band of graphene that has a binding energy of more than 3.5 eV. We argue that the observed kinks are indeed caused by the electron-phonon interaction, but the role of the Fermi-Dirac distribution cutoff is assumed by a cut-off in the density of $σ$ states. The existence of the effect suggests a very weak coupling of holes in the $σ$ band not only to the $π$ electrons of graphene but also to the substrate electronic states. This is confirmed by the presence of such kinks for graphene on several different substrates that all show a strong coupling constant of lambda=1.

cond-mat.mes-hall↗

arXiv2013

Hydrodynamic theory of coupled current and magnetization dynamics in spin-textured antiferromagnets

Antiferromagnets with vanishingly small (or zero) magnetization are interesting candidates for spintronics applications. In the present paper we propose two models for description of the current-induced phenomena in antiferromagnetic textures. We show that the magnetization that originates from rotation or oscillations of antiferromagnetic vector can, via $sd$-exchange coupling, polarize the current and give rise to adiabatic and nonadiabatic spin torques. Due to the Lorentz-type dynamics of antiferromagnetic moments (unlike the Galilenian-like dynamics in ferromagnets), the adiabatic spin torque affects the characteristic lengthscale of the moving texture. Nonadiabatic spin torque contributes to the energy pumping and can induce the stable motion of antiferromagnetic texture, but, in contrast to ferromagnets, has pure dynamic origin. We also consider the current-induced phenomena in artificial antiferromagnets where the current maps the staggered magnetization of the structure. In this case the effect of nonadiabatic spin torque is similar to that in ferromagnetic constituents of the structure. In particular, the current can remove degeneracy of the translational antiferromagnetic domains indistinguishable in the external magnetic field and thus can set into motion the 180$^\circ$ domain wall.

cond-mat.mes-hall↗

arXiv2013

Control of the Exciton Transfer in Quantum Dots by the Stark Effect

Resonant transfer of energy between excited states in a system of two semiconductor quantum dots is studied theoretically. The model Hamiltonian has been formulated, which allowed describe the impact on the dynamics of the resonant laser pulse, the Coulomb interaction, the static Stark effect, and the relaxation of the exciton states. Examples of calculations for efficiency of the energy transfer under different excitation conditions are presented. The control of the process by the level shift in a constant electric field is demonstrated.

cond-mat.mes-hall↗