arXiv subjects

P. Singha Deo

Publications and source records attributed to P. Singha Deo.

At least 37 records · Page 2Linked to original sources

arXiv2005

Larmor precession time, Wigner delay time and the local density of states in a quantum wire

Buttiker-Thomas-Pretre (BTP) [Z. Phys B {\bf 94}, 133 (1994)] proposed that the concepts behind the Larmor clock tell us that it is possible to define exactly the local density of states (LDOS) in terms of the scattering matrix. However, we take into account evanescent modes and show that for an impurity in a quantum wire, this is in principle not exactly true. We also prove that the Wigner delay time gives correct superluminal times at the Fano resonances, in spite of the fact that the stationary phase approximation is not valid there.

cond-mat.mes-hall↗

arXiv2005

Importance of individual scattering matrix elements at Fano resonances

Single particle resonances in quantum wires are generally Fano resonances. In case of Fano resonances, the scattering phase shift in some channels show sharp phase drops and that in the other channels do not. Phase shift in a particular channel can be measured and can yield information about the integrated charge localized around the scatterer. This paper tries to analyze if some channels are more informative than the others, so that an experimentalist can measure the phase shift in only those channels.

cond-mat.mes-hall↗

arXiv2004

On the structure and spectrum of classical two-dimensional clusters with a logarithmic interaction potential

We present a numerical study of the effect of the repulsive logarithmic inter-particle interaction on the ground state configuration and the frequency spectrum of a confined classical two-dimensional cluster containing a finite number of particles. In the case of a hard wall confinement all particles form one ring situated at the boundary of the potential. For a general r^n confinement potential, also inner rings can form and we find that all frequencies lie below the frequency of a particular mode, namely the breathing-like mode. An interesting situation arises for the parabolic confined system(i.e. n=2). In this case the frequency of the breathing mode is independent of the number of particles leading to an upper bound for all frequencies. All results can be understood from Earnshaw's theorem in two dimensions. In order to check the sensitivity of these results, the spectrum of vortices in a type II superconductor which, in the limit of large penetration depths, interact through a logarithmic potential, is investigated.

cond-mat.str-el↗

arXiv2003

Quantum current magnification in a multi-channel mesoscopic ring

We have studied the current magnification effect in a multi-channel open mesoscopic ring. We show that the current magnification effect is robust even in the presence of several propagating modes inspite of mode mixing and cancellation effects. The magnitude of circulating currents in the multi-channel regime can be much larger than that in a single channel case. Impurities can enhance or degrade the current magnification effect depending sensitively on the system parameters. Circulating currents are mostly associated with Fano resonances in the total transport current. We further show that system-lead coupling qualitatively changes the current magnification effect.

cond-mat.mes-hall↗

arXiv2003

Energy spectrum, persistent current and electron localization in quantum rings

Energy spectra of quasi-one-dimensional quantum rings with a few electrons are studied using several different theoretical methods. Discrete Hubbard models and continuum models are shown to give similar results governed by the special features of the one-dimensionality. The energy spectrum of the many-body system can be described with a rotation-vibration spectrum of a 'Wigner molecule' of 'localized' electrons, combined with the spin-state determined from an effective antiferromagnetic Heisenberg Hamiltonian. The persistent current as a function of magnetic flux through the ring shows periodic oscillations arising from the 'rigid rotation' of the electron ring. For polarized electrons the periodicity of the oscillations is always the flux quantum Phi_0. For nonpolarized electrons the periodicity depends on the strength of the effective Heisenberg coupling and changes from Φ_0 first to Phi_0/2 and eventually to Phi_0/N when the ring gets narrower.

cond-mat.mes-hall↗

arXiv2003

Quantum rings for beginners: Energy spectra and persistent currents

Theoretical approaches to one-dimensional and quasi-one-dimensional quantum rings with a few electrons are reviewed. Discrete Hubbard-type models and continuum models are shown to give similar results governed by the special features of the one-dimensionality. The energy spectrum of the many-body states can be described by a rotation-vibration spectrum of a 'Wigner molecule' of 'localized' electrons, combined with the spin-state determined from an effective antiferromagnetic Heisenberg Hamiltonian. The persistent current as a function of the magnetic flux through the ring shows periodic oscillations arising from the 'rigid rotation' of the electron ring. For polarized electrons the periodicity of the oscillations is always the flux quantum Phi_0. For nonpolarized electrons the periodicity depends on the strength of the effective Heisenberg coupling and changes from Phi_0 first to Phi_0/2 and eventually to Phi_0/N when the ring gets narrower.

cond-mat.mes-hall↗

arXiv2003

Friedel Sum Rule for single channel quantum wire

Elastic scattering in a quantum wire has several novel features not seen in 1D, 2D or 3D. In this work we consider a single channel quantum wire as its application is inevitable in making devices based on quantum interference effects. We consider a point defect or a single delta function impurity in such a wire and show how some of these novel features affect Friedel-sum-rule (FSR) in a way, that is quite unlike in 1D, 2D and 3D.

cond-mat.mes-hall↗

arXiv2003

Fractional Periodicity of Persistent Currents: A Signature of Broken Internal Symmetry

We show from the symmetries of the many body Hamiltonian, cast into the form of the Heisenberg (spin) Hamiltonian, that the fractional periodicities of persistent currents are due to the breakdown of internal symmetry and the spin Hamiltonian holds the explanation to this transition. Numerical diagonalizations are performed to show this explicitely. Persistent currents therefore, provide an easy way to experimentally verify broken internal symmetry in electronic systems.

cond-mat.mes-hall↗

arXiv2002

Scattering phase shifts in quasi-one-dimension

Scattering of an electron in quasi-one dimensional quantum wires have many unusual features, not found in one, two or three dimensions. In this work we analyze the scattering phase shifts due to an impurity in a multi-channel quantum wire with special emphasis on negative slopes in the scattering phase shift versus incident energy curves and the Wigner delay time. Although at first sight, the large number of scattering matrix elements show phase shifts of different character and nature, it is possible to see some pattern and understand these features. The behavior of scattering phase shifts in one-dimension can be seen as a special case of these features observed in quasi-one-dimensions. The negative slopes can occur at any arbitrary energy and Friedel sum rule is completely violated in quasi-one-dimension at any arbitrary energy and any arbitrary regime. This is in contrast to one, two or three dimensions where such negative slopes and violation of Friedel sum rule happen only at low energy where the incident electron feels the potential very strongly (i.e., there is a very well defined regime, the WKB regime, where FSR works very well). There are some novel behavior of scattering phase shifts at the critical energies where $S$-matrix changes dimension.

cond-mat.mes-hall↗

arXiv2001

Electron correlation effects in the presence of non-symmetry dictated nodes

We numerically study the effect of non-symmetry-dictated-nodes (NSDN) on electron correlation effects for spinless electrons. We find that repulsive interaction between electrons can enhance the overlap between nearest neighbors in the tight binding Hamiltonian, in the presence of NSDN. Normally, in the absence of NSDN, attractive interaction between electrons give such an effect and repulsive interaction gives opposite effect.

cond-mat.mes-hall↗

arXiv2000

Violation of general Friedel sum rule in mesoscopic systems

In the wake of a new kind of phase generally occurring in mesoscopic transport phenomena, we discuss the validity of Friedel sum rule in the presence of this phase. We find that the general Friedel sum rule may be violated.

cond-mat.mes-hall↗

arXiv2000

Temperature enhanced persistent currents and "$ϕ_0/2$ periodicity"

We predict a non-monotonous temperature dependence of the persistent currents in a ballistic ring coupled strongly to a stub in the grand canonical as well as in the canonical case. We also show that such a non-monotonous temperature dependence can naturally lead to a $ϕ_0/2$ periodicity of the persistent currents, where $ϕ_0$=h/e. There is a crossover temperature $T^*$, below which persistent currents increase in amplitude with temperature while they decrease above this temperature. This is in contrast to persistent currents in rings being monotonously affected by temperature. $T^*$ is parameter-dependent but of the order of $Δ_u/π^2k_B$, where $Δ_u$ is the level spacing of the isolated ring. For the grand-canonical case $T^*$ is half of that for the canonical case.

cond-mat.mes-hall↗

arXiv2000

Novel interference effects and a new Quantum phase in mesoscopic systems

Mesoscopic systems have provided an opportunity to study quantum effects beyond the atomic realm. In these systems quantum coherence prevails over the entire sample. We discuss several novel effects related to persistent currents in open systems which do not have analogues in closed systems. Some phenomena arising simultaneously due to two non-classical effects namely, Aharonov-Bohm effect and quantum tunneling are presented. Simple analysis of sharp phase jumps observed in double-slit Aharonov-Bohm experiments is given. Some consequences of parity violation are elaborated. Finally, we briefly describe the dephasing of Aharonov-Bohm oscillations in Aharonov-Bohm ring geometry due to spin-flip scattering in one of the arms. Several experimental manifestations of these phenomena and their applications are given.

cond-mat.mes-hall↗

arXiv2000

Current-spin-density functional study of persistent currents in quantum rings

We present a numerical study of persistent currents in quantum rings using current spin density functional theory (CSDFT). This formalism allows for a systematic study of the joint effects of both spin, interactions and impurities for realistic systems. It is illustrated that CSDFT is suitable for describing the physical effects related to Aharonov-Bohm phases by comparing energy spectra of impurity-free rings to existing exact diagonalization and experimental results. Further, we examine the effects of a symmetry-breaking impurity potential on the density and current characteristics of the system and propose that narrowing the confining potential at fixed impurity potential will suppress the persistent current in a characteristic way.

cond-mat.mes-hall↗

arXiv2000

Heat Capacity of Mesoscopic Superconducting Disks

We study the heat capacity of isolated giant vortex states, which are good angular momentum ($L$) states, in a mesoscopic superconducting disk using the Ginzburg-Landau (GL) theory. At small magnetic fields the $L$=0 state qualitatively behaves like the bulk sample characterized by a discontinuity in heat capacity at $T_c$. As the field is increased the discontinuity slowly turns into a continuous change which is a finite size effect. The higher $L$ states show a continuous change in heat capacity at $T_c$ at all fields. We also show that for these higher $L$ states, the behavior of the peak position with change in field is related to the paramagnetic Meissner effect (irreversible) and can lead to an unambiguous observation of positive magnetization in mesoscopic superconductors.

cond-mat.mes-hall↗

arXiv2000

Features of level broadening in a ring-stub system

When a one dimensional (1D) ring-stub system is coupled to an electron reservoir, the states acquire a width (or broadening characterized by poles in the complex energy plane) due to finite life time effects. We show that this broadening is limited by anti-resonances due to the stub. The differences in level broadening in presence and absence of anti-resonance is exemplified by comparison to a 1D ring coupled to an infinite reservoir. We also show that the anti-resonances due to the stub has an anchoring effect on the poles when a magnetic flux through the ring is varied. This will have implication on change in distribution of the poles in disordered multichannel situation as magnetic flux is varied.

cond-mat.mes-hall↗

arXiv1999

Comment on ``Friedel phases and phases of transmission amplitudes in quantum scattering systems" by T. Taniguchi and M. Buettiker

We take a modified boundary condition at the dead end of a stub to simulate transmission zeroes being replaced by minima and then the discontinuous phase slip (or decrease) at the transmission zeroes are replaced by a continuous but rapid phase slip. The modified boundary condition can be continuously tuned to give the results of the stub with hard wall boundary condition at the dead end of stub. Even when the phase slip is continuous one can obtain information about the density of states in the stub region from the scattering phases.

cond-mat↗

arXiv1998

Mesoscopic superconducting disks

Using the non-linear Ginzburg-Landau (GL) eqs. type I superconducting disks of finite radius ($R$) and thickness ($d$) are studied in a perpendicular magnetic field. Depending on $R$ and $d$, first or second order phase transitions are found for the normal to superconducting state. For sufficiently large $R$ several transitions in the superconducting phase are found corresponding to different angular momentum giant vortex states. In increasing magnetic field the superconductor is in its ground state, while in field down sweep it is possible to drive the system into metastable states. We also present a quantitative analysis of the relation between the detector output and the sample magnetization. The latter, and the incorporation of the finite thickness of the disks, are essential in order to obtain quantitative agreement with experiment.

cond-mat.mes-hall↗