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At least 73 records · Page 4Linked to original sources
arXiv2007

Magnetic fingerprint in a ferromagnetic wire: Spin torque diode effect and induction of the DC voltage spectrum inherent in the wire under application for RF current

We report the rectifying effect of a constant-wave radio frequency (RF) current by a magnetic domain wall (DW) on a single-layered ferromagnetic wire. A direct-current (DC) voltage is generated by the spin torque diode effect, which is a consequence of magnetoresistance oscillation due to the resonant spin wave excitation induced by the spin-polarized RF current. The DC voltage spectrum strongly depends on the internal spin structure in the DW, which corresponds to the magnetic fingerprint of the spin structure in the ferromagnetic wire.

cond-mat.mtrl-sci↗
arXiv2007

Enhanced decoherence in the vicinity of a phase transition

We study the decoherence of a spin-1/2 induced by an environment which is on the verge of a continuous phase transition. We consider spin environments described by the ferromagnetic and antiferromagnetic Heisenberg models on a square lattice. As is well known, these two dimensional systems undergo a continuous phase transition at zero temperature, where, the spins order spontaneously. For weak coupling of the central spin to these baths, we find that as one approaches the transition temperature, critical fluctuations make the central spin decohere faster. Furthermore, the decoherence is maximal at zero temperature as signalled by the divergence of the Markovian decoherence rate.

cond-mat.mes-hall↗
arXiv2007

Realizable spin models and entanglement dynamics in superconducting flux qubit systems

Realizable spin models are investigated in a two superconducting flux qubit system. It is shown that a specific adjustment of system parameters in the two flux qubit system makes it possible to realize an artificial two-spin system that cannot be found naturally. For the artificial two-spin systems, time evolution of a prepared quantum state is discussed to quantify quantum entanglement dynamics. The concurrence and fidelity as a function of time are shown to reveal a characteristic entanglement dynamics of the artificial spin systems. It is found that the unentangled input state can evolute to be a maximally entangled output state periodically due to the exchange interactions induced by two-qubit flipping tunneling processes while single-qubit flipping tunneling processes plays a role of magnetic fields for the artificial spins.

cond-mat.mes-hall↗
arXiv2007

Tunneling into low-dimensional and strongly correlated conductors

A general nonperturbative theory of the low-energy electron propagator is developed and used to calculate the single-particle density of states in a variety of systems. This method involves the decoupling of the electron-electron interaction through a Hubbard-Stratonovich transformation, followed by a saddle-point approximation of the remaining functional integral. The final expression is found to be the tunneling analog of the infrared catastrophe that occurs in the x-ray edge problem; here, the host system responds to the potential produced by the abrupt addition of an electron during a tunneling event. This response can lead to a suppression in the tunneling density of states near the Fermi energy. This method is adaptable to lattice or continuum models of any dimensionality, with or without translational invariance. When applied, the exact density of states is obtained for the Tomonaga-Luttinger model, and the pseudogap of a fractional quantum Hall fluid is recovered.

cond-mat.str-el↗
arXiv2007

Strong g-Factor Anisotropy in Hole Quantum Dots Defined in Ge/Si Nanowires

We demonstrate fully tunable single and double quantum dots in a one-dimensional hole system based on undoped Ge/Si core-shell nanowire heterostructures. The local hole density along the nanowire is controlled by applying voltages to five top gate electrodes with a periodicity of 80 nm, insulated from the wire by a 20 nm-thick HfO_2 dielectric film. Low-temperature transport measurements were used to investigate the magnetic field dependence of Coulomb blockade peaks in a single quantum dot and indicate a strongly anisotropic g-factor with |g_para| = 0.60 +/- 0.03 and |g_perp| < 0.12.

cond-mat.mes-hall↗
arXiv2007

Terahertz Parametric Gain in Semiconductor Superlattices

We consider a high-frequency response of electrons in a single miniband of superlattice subject to dc and ac electric fields. Action of ac electric field causes oscillations of electron's effective mass in miniband, which result in a parametric resonance. We have established a theoretical feasibility of phase-sensitive parametric amplification at the resonance. The parametric amplification does not require operation in conditions of negative differential conductance. Therefore a formation of destructive domains of high electric field inside the superlattice can be prevented. Here we concentrate on the parametric up- and down-conversion of electromagnetic radiation from available frequencies to desirable THz frequency range.

cond-mat.mes-hall↗
arXiv2007

Reconstruction of Quantum Well Potentials via the Intertwining Operator Technique

One of the most important issues of quantum engineering is the construction of low-dimensional structures possessing desirable properties. For example, in different areas of possible applications of the structures containing quantum wells (QW), there is need to have QW energy spectrum, which is predetermined. Then the following question arises: can one reconstruct the shape of QW which supports this spectrum? We outline the possible strategy of the QW potential shape reconstruction, if the spectrum of QW is given in advance. The proposed approach is based on the combination of different techniques such as Inverse Scattering Problem Method, Darboux and Liouville transformation. It enables to take into account the space-variable dependent effective mass of charge carriers and allows the kinetic energy operator to be of non-Hermitian as well as Hermitian form. The proposed technique allows to construct phase-equivalent potentials, to add the new bounded states to (or remove some of them from) the spectrum supported by an initial potential and provides a systematic procedure for generating new exactly solvable models.

cond-mat.mes-hall↗
arXiv2007

Symmetry breaking by the sea of Dirac-Landau levels in graphene

The quantum Hall states of graphene have a filled Dirac sea of Landau levels. The short ranged SU(4) symmetry breaking interactions can induce a staggered polarization of the sea of Dirac-Landau levels. We study this effect in the extended Hubbard model on a honeycomb lattice using mean field variational wavefunctions. We find a valley symmetry broken, anti-ferromagnetic spin ordered phase at $\nu=\pm 1$ when the on-site interaction is dominant. Our mean field solution is consistent with the recently reported experimental results of Z. Jiang et. al.\cite{jiang}

cond-mat.mes-hall↗
arXiv2007

Dynamical and scaling properties of nu=5/2 interferometer

We calculate the non-linear I-V tunneling curves for a two-point-contact tunneling junction between two edges of the $\nu={5/2}$ non-abelian fractional quantum Hall state. The non-linear I-V tunneling curves are calculated for both cases with and without an $e/4$ non-abelian quasiparticle between the two contacts. We confirm that, within a dynamical edge theory, the presence of the $e/4$ quasiparticle between the two contacts destroys the interference between the two tunneling paths. We also calculate how the interference reappears as the $e/4$ quasiparticle is moved closer to an edge.

cond-mat.mes-hall↗
arXiv2007

Resistance without resistors: An anomaly

The elementary 2-terminal network consisting of a resistively ($R-$) shunted inductance ($L$) in series with a capacitatively ($C-$) shunted resistance ($R$) with $R = \sqrt{L/C}$, is known for its non-dispersive dissipative response, $i.e.,$ with the input impedance $Z_0(\omega) = R$, independent of the frequency ($\omega$). In this communication we examine the properties of a novel equivalent network derived iteratively from this 2-terminal network by replacing everywhere the elemental resistive part $R$ with the whole 2-terminal network. This replacement suggests a recursion $Z_{n+1}(\omega) = f(Z_n(\omega))$, with the recursive function $f(z) = (i\omega Lz/i\omega L + z) + (z/1+i\omega Cz)$. The recursive map has two fixed points -- an unstable fixed point $Z_u^\star = 0$, and a stable fixed point $Z_s^\star = R$. Thus, resistances at the boundary terminating the infinitely iterated network can now be made arbitrarily small without changing the input impedance $Z_\infty (= R)$. This, therefore, leads to realizing in the limit $n\to\infty$ an effectively dissipative network comprising essentially non-dissipative reactive elements ($L$ and $C$) only. Hence the oxymoron -- resistance without resistors! This is best viewed as a classical anomaly akin to the one encountered in turbulence. Possible application as a formal decoherence device -- the {\it fake channel} -- is briefly discussed for its quantum analogue.

cond-mat.dis-nn↗
arXiv2007

Transport through single-level quantum dot in a magnetic field

We study the effect of an external magnetic field on the transport properties of a quantum dot using a recently developed extension of the functional renormalization group approach to non-equilibrium situations. We discuss in particular the interplay and competition of the different energy scales of the dot and the magnetic field on the stationary non-equilibrium current and conductance. As rather interesting behavior we find a switching behavior of the magnetic field for intermediate correlations and bias voltage.

cond-mat.mes-hall↗
arXiv2007

Interference pattern of a long diffusive Josephson junction

We calculate the modulation by a magnetic field of the critical current of a long disordered Josephson junction in the diffusive limit, i.e. when the dimensions of the junction are larger that the elastic mean free path, and when the length $L$ is much larger than the width $w$. Due to the averaging of the gauge invariant phase factor over diffusive trajectories, the well-known oscillations of the Fraunhofer pattern are smoothed out and replaced by an exponential decay at large field. The predicted pattern is universal, i.e., it is independent of the disorder strength. We point out an interesting relation with the physics of speckle correlations in optics of turbid media.

cond-mat.mes-hall↗
arXiv2007

Spacial Modulation of the Magnetization in Cobalt Nanowires

Cobalt nanowires with a diameter in the range between 50 to 100nm can be prepared as single-crystal wires with the easy axis (the c-axis) perpendicular to the wire axis. The competition between the crystal anisotropy and demagnetization energy frustrates the magnetization direction. A periodic modulation of the angle between M and the wire axis yields a lower energy.

cond-mat.mes-hall↗
arXiv2007

Kinetics of Electrodeposition of Silver and Copper at Template Synthesis of Nanowires

The results of investigation of kinetics of nanopores filling into membranes from aluminum oxide (pore diameter - 200 nm, porosity ~ 50%) at electrodeposition of copper and silver are described. It is shown, that at identical quantity of electricity passed through solutions, the degree of pores filling by metal (average thickness of a deposit) is various for copper and silver deposition. Calculated (according Faraday Law) and experimental dependences of deposition rates of these metals on quantity of electricity passed at direct and pulse currents are presented. Galvanodynamic i - v dependences have been obtained at various current scanning rates. The smaller rate of deposition allows to decrease concentration limitations of electrode process and to obtain higher average thickness of metal deposits and higher filling degree. The limiting values of quantity of electricity for direct and pulse currents were determined. The average thickness of silver and copper deposits was obtained. A degree of pores filling, the morphology and chemical microanalysis were studied on cross-section of the membrane, using TESCAN SEM equipped with an Oxford Instruments INCA Enerqy EDX-system.

cond-mat.mes-hall↗
arXiv2007

Tensoresistive Effect in Single Crystal Microwires of Pbte Doped with Tl

Results of room temperature measurements of tensoresistive effect of thin single crystal microwires of Pb1-xTlxTe (x=0.0000 - 0.0025, d = 5 - 20 micrometers) obtained from the melted compound of corresponding composition by the filling of quartz capillary with the following crystallization of material are presented. For the samples corresponding to chemical composition with concentration of thallium x ~0,0025 an essential increase of tensoresistive effect (resistance changes for elastic elongations per unit length of a crystal) in comparison with nondoped samples is observed. Various mechanisms which can lead to observable anomalies, including resonance scattering are discussed. Obtained experimental results allow us to suppose that the observed peculiarities can be interpreted on the basis of model of an impurity band of Tl in PbTe.

cond-mat.mtrl-sci↗
arXiv2007

Inducement and suppression of Coulomb effects in elastic 2D-2D electron tunnelling in a quantizing magnetic field

Tunnelling between two-dimensional electron systems has been studied in the magnetic field perpendicular to the systems planes. The satellite conductance peaks of the main resonance have been observed due to the electron tunnelling assisted by the elastic scattering on impurities in the barrier layer. These peaks are shown to shift to the higher voltage due to the Coulomb pseudogap in the intermediate fields. In the high magnetic fields the pseudogap shift is disappeared.

cond-mat.mes-hall↗