SEARCH · Searcharxiv

Results for “cond-mat.mes-hall”

Search indexed arXiv papers on artificial intelligence, large language models, computer vision and robotics. Read source abstracts and follow links to arXiv.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 109 records · Page 6Linked to original sources

arXiv2008

Geometric effects of noise on Cooper pair pumping

Several works investigated the connection between Cooper pair pumping and the geometric and topological properties of the pumping cycle in the parameters space. Recently the predicted connection between Berry's phase and pumped charge was observed experimentally. In the present work we analyze the effects of noise on Cooper pair pumping. We show that dissipation induces a new geometric contribution in the transferred charge and we propose an experiment where the theory can be probed.

cond-mat.mes-hall↗

arXiv2008

Comment on: Quantum interference of tunnel trajectories between states of different spin length in a dimeric molecular nanomagnet

Ramsey et al. [Nature Phys. 4, 277-281 (2008)] report the observation of quantum interference associated with tunnelling trajectories between states of different total spin length in a dimeric molecular nanomagnet. They argue that the interference is a consequence of the unique characteristics of a molecular Mn12 wheel, which behaves as a molecular dimer with weak ferromagnetic exchange coupling. We show here that the data published by Ramsey et al. are not consistent and unfortunately mostly wrong. We show further that the Landau-Zener (LZ) formula, which links the tunnel probability with the tunnel splitting, can only be applied in a well-defined experimental region, which lays outside the region accessed by Ramsey and colleagues. Only a lower-limit estimate of the tunnel splitting can be obtained, showing that the observed transition cannot be explained with the dimer model. We also present other shortcomings of the paper questioning the dimer model, and that the alignment of the magnetic field is crucial for observing quantum interference.

cond-mat.mes-hall↗

arXiv2008

Chiral tunneling through time-periodic potential in graphene

Chiral tunneling through a harmonically driven potential barrier in graphene monolayer is considered in this work. Since the quasiparticles in this system are chiral in nature, tunneling is highly anisotropic, we determine the transmission probabilities for the central and sidebands as the incident angle of the electron beam is changed . Furthermore, we investigate how the transmission probabilities change as the width, amplitude and frequency of the oscillating barrier is changed. An interesting result of our study is that perfect transmission for normal incidence that has been reported for a static barrier persists for the oscillating barrier, manifestation of Klein tunneling in a time harmonic potential.

cond-mat.mes-hall↗

arXiv2009

Single-band tight-binding parameters for Fe-MgO-Fe magnetic heterostructures

We present a computationally efficient transferable single-band tight-binding model (SBTB) for spin polarized transport in heterostructures with an effort to capture the band structure effects. As an example, we apply it to study transport through Fe-MgO-Fe(100) magnetic tunnel junction devices. We propose a novel approach to extract suitable tight-binding parameters for a material by using the energy resolved transmission as the benchmark, which inherently has the bandstructure effects over the two dimensional transverse Brillouin zone. The SBTB parameters for each of the four symmetry bands for bcc Fe(100) are first proposed which are complemented with the transferable tight-binding parameters for the MgO tunnel barrier for the Delta_1 and Delta_5 bands. The non-equilibrium Green's function formalism is then used to calculate the transport. Features like I-V characteristics, voltage dependence and the barrier width dependence of the tunnel magnetoresistance ratio are captured quantitatively and the trends match well with the ones observed by ab initio methods.

cond-mat.mes-hall↗

arXiv2008

Non-equilibrium Green's function based single-band tight-binding model for Fe-MgO-Fe magnetic tunnel junction devices

Motivated by observation of very high tunnel magnetoresistance (TMR) in Fe-MgO-Fe magnetic tunnel junction devices, we propose a theoretical model for these devices based on a single-band tight-binding approximation. An effort is made to capture the band dispersions over the two dimensional transverse Brillouin zone. In the transport direction, spin dependent Hamiltonian is prescribed for Delta_1 and Delta_5 bands. Non-equilibrium Green's function formalism is then used to calculate transport. Features like voltage dependence of TMR are captured quantitatively within this simple model and the trends match well with the ones predicted by ab-initio methods and experiments.

cond-mat.mes-hall↗

arXiv2008

Quantum modulation of the Kondo resonance of Co adatoms on Cu/Co/Cu(100)

Low-temperature scanning tunneling spectroscopy reveals that the Kondo temperature T_K of Co atoms adsorbed on Cu/Co/Cu(100) multilayers varies between 60 K and 134 K as the Cu film thickness decreases from 20 to 5 atomic layers. The observed change of T_K is attributed to a variation of the density of states at the Fermi level ρ_F induced by quantum well states confined to the Cu film. A model calculation based on the quantum oscillations of ρ_F at the belly and the neck of the Cu Fermi surface reproduces most of the features in the measured variation of T_K.

cond-mat.mes-hall↗

arXiv2008

Site-selective conductance of sidewall functionalized carbon nanotubes

We use DFT to study the effect of molecular adsorbates on the conductance of metallic carbon nanotubes. The five molecules considered (NO2, NH2, H, COOH, OH) lead to similar scattering of the electrons. The adsorption of a single molecule suppresses one of the two available channels of the CNT at low bias conductance. If more molecules are adsorbed on the same sublattice, the remaining open channel can be blocked or not, depending on the relative position of the adsorbates. If a simple geometric condition is fulfilled this channel is still open, even after adsorbing an arbitrary number of molecules.

cond-mat.mes-hall↗

arXiv2008

Comment on "Superinsulator and Quantum Synchronization"

We show that the "theory" of "superinsulating" state presented by V.Vinokur et al (Nature vol. 452, p. 613, 2008) and Fistul et al (Phys. Rev. Lett. vol. 100, 086805, 2008) is essentially incorrect due to a sequence of errors in the theoretical analysis of the standard model of Josephson arrays which properties have been carefully studied and described in the literature. The line of calculations suggested in these articles lead to unphysical results. In particular, the calculations predict a direct current flowing through a capacitor. Moreover, this current may flow even in the absence of voltage - a sort of supercurrent flowing in the "superinsulating" state. We also question that the theoretical model employed in these works, even if treated correctly, is applicable to the analysis of experimental data on homogeneously disordered superconductive films.

cond-mat.supr-con↗

arXiv2012

Electronic structure of magnetically modulated graphene

We present a theoretical study of the electronic structure of magnetically modulated graphene. We consider monolayer graphene in the presence of a perpendicular magnetic field and a unidirectional weak magnetic modulation. The density of states and the bandwidth of the Dirac electrons in this system are determined. We have found magnetic Weiss oscillations in the bandwidth and the density of states. These oscillations are out of phase and larger in amplitude than the ones in the electrically modulated graphene. In addition, these oscillations are in phase and smaller in amplitude to those of magnetically modulated standard electron gas system.

cond-mat.mes-hall↗

arXiv2008

Resistance Quenching in Graphene Interconnects

We investigated experimentally the high-temperature electrical resistance of graphene interconnects. The test structures were fabricated using the focused ion beam from the single and bi-layer graphene produced by mechanical exfoliation. It was found that as temperature increases from 300 to 500K the resistance of the single- and bi-layer graphene interconnects drops down by 30% and 70%, respectively. The quenching and temperature dependence of the resistance were explained by the thermal generation of the electron-hole pairs and acoustic phonon scattering. The obtained results are important for the proposed applications of graphene as interconnects in integrated circuits.

cond-mat.mtrl-sci↗

arXiv2008

Fluctuation relations without microreversibility in nonlinear transport

In linear transport, the fluctuation-dissipation theorem relates equilibrium current correlations to the linear conductance coefficient. For nonlinear transport, there exist fluctuation relations that rely on Onsager's principle of microscopic reversibility away from equilibrium. However, both theory and experiments have shown deviations from microreversibility in the form of magnetic field asymmetric current-voltage relations. We present novel fluctuation relations for nonlinear transport in the presence of magnetic fields that relate current correlation functions at any order at equilibrium to response coefficients of current cumulants of lower order. We illustrate our results with the example of an electrical Mach-Zehnder interferometer.

cond-mat.mes-hall↗

arXiv2008

Calculated magneto-resistance due to domain walls : the role of impurity scattering

The existing Levy-Zhang approach to constructing the contribution to the resistivity of a magnetic domain wall is explored. The model equations are integrated analytically, giving a closed form expression for the resistivity when the current flows in the wall. The Boltzmann equation is solved analytically and the ratio of the spin up and spin down resistivities is calculated and its dependence on the strength of the Coulomb and exchange scattering potentials is elucidated.

cond-mat.mes-hall↗

arXiv2008

Winding up by a quench: Insulator to superfluid phase transition in a ring of BECs

We study phase transition from the Mott insulator to superfluid in a periodic optical lattice. Kibble-Zurek mechanism predicts buildup of winding number through random walk of BEC phases, with the step size scaling as a the third root of transition rate. We confirm this and demonstrate that this scaling accounts for the net winding number after the transition.

cond-mat.stat-mech↗

arXiv2008

The decoherence of coupled spin qubits system

In this report, we examine the decoherence of a spin qubits system coupled to counted spin chain with a $1/r^2$ interaction by using influence functional. We also examine the time evolution of density matrix numerically when environment is Gaussian noise.

cond-mat.mes-hall↗

arXiv2008

Density of states of disordered graphene

We calculate the average single particle density of states in graphene with disorder due to impurity potentials. For unscreened short-ranged impurities, we use the non-self-consistent and self-consistent Born and $T$-matrix approximations to obtain the self-energy. Among these, only the self-consistent $T$-matrix approximation gives a non-zero density of states at the Dirac point. The density of states at the Dirac point is non-analytic in the impurity potential. For screened short-ranged and charged long-range impurity potentials, the density of states near the Dirac point typically increases in the presence of impurities, compared to that of the pure system.

cond-mat.mes-hall↗

arXiv2009

Period Doubling in Small Multiply Connected Superconductors

It is shown that for superconductors with circumference $2πR$ approaching BCS coherence length $ξ_0$ minimal period of the response of all thermodynamic quantities to external magnetic field is set by $hc/e$ i.e. twice the corresponding value for the bulk case. This is explained by the dependence of internal energy of Cooper pairs on their center of mass motion which leads, in particular, to a transition offset between different current-carrying states. Explicit calculation of the transition offset is done for the case of s-wave superconducting cylinder with $R \gg ξ_0$ and turns out to be exponentially small. A possible enhancement of the effect for nodal superconductors is suggested. Similar conclusions should also apply to the response of charged or neutral superfluids to rotation.

cond-mat.supr-con↗

arXiv2009

Analog information processing at the quantum limit with a Josephson ring modulator

Amplifiers are crucial in every experiment carrying out a very sensitive measurement. However, they always degrade the information by adding noise. Quantum mechanics puts a limit on how small this degradation can be. Theoretically, the minimum noise energy added by a phase preserving amplifier to the signal it processes amounts at least to half a photon at the signal frequency. In this article, we show that we can build a practical microwave device that fulfills the minimal requirements to reach the quantum limit. This is of importance for the readout of solid state qubits, and more generally, for the measurement of very weak signals in various areas of science. We also discuss how this device can be the basic building block for a variety of practical applications such as amplification, noiseless frequency conversion, dynamic cooling and production of entangled signal pairs.

cond-mat.mes-hall↗

arXiv2010

Solvable Limit for SU(N) Kondo Model

We study a single channel one dimensional Kondo Model where the impurity spin is replaced by an su(n) spin. Using Abelian bosonization and canonical transformation we explicitly show that this system has an exactly solvable point. The calculation also shows that there are $n$ collective excitation modes in the system, one charged and $n-1$ neutral spin excitation modes.

cond-mat.mes-hall↗