arXiv subjects

P. Singha Deo

Publications and source records attributed to P. Singha Deo.

51 records · Page 3Linked to original sources

arXiv1998

Vortex phase diagram for mesoscopic superconducting disks

Solving numerically the 3D non linear Ginzburg-Landau (GL) equations, we study equilibrium and nonequilibrium phase transitions between different superconducting states of mesoscopic disks which are thinner than the coherence length and the penetration depth. We have found a smooth transition from a multi-vortex superconducting state to a giant vortex state with increasing both the disk thickness and the magnetic field. A vortex phase diagram is obtained which shows, as function of the magnetic field, a re-entrant behavior between the multi-vortex and the giant vortex state.

cond-mat.supr-con↗

arXiv1998

Hysteresis in mesoscopic superconducting disks: the Bean-Livingston barrier

The magnetization behavior of mesoscopic superconducting disks can show hysteretic behavior which we explain by using the Ginzburg-Landau (GL) theory and properly taking into account the de-magnetization effects due to geometrical form factors. In large disks the Bean-Livingston surface barrier is responsible for the hysteresis. While in small disks a volume barrier is responsible for this hysteresis. It is shown that although the sample magnetization is diamagnetic (negative), the measured magnetization can be positive at certain fields as observed experimentally, which is a consequence of the de-magnetization effects and the experimental set up.

cond-mat.supr-con↗

arXiv1997

Magnetization of mesoscopic superconducting discs

Solutions of Ginzburg-Landau eqns. coupled with three dimensional Maxwell eqns. reveal intriguing magnetic response of small superconducting particles, qualitatively different from the two dimensional approximation but in agreement with recent experiments. Depending on the radius and thickness first or second order transitions are found for the normal to superconducting state. For a sufficient large radius of the disc several transitions in the superconducting phase are obtained which correspond to different angular momentum giant vortex states. The incorporation of the finite thickness in the calculation is crucial in order to obtain agreement with the position and the size of these jumps, and the line shape and magnitude of the magnetization curves.

cond-mat.supr-con↗

arXiv1997

Conductance Quantization in a Periodically Modulated Quantum Channel: backscattering and mode mixing

It is known that the conductance of a quantum point contact is quantized in units of $2e^2/h$ and this quantization is destroyed by a non-adiabatic scatterer in the point contact, due to backscattering. Recently, it was shown [Phys. Rev. Lett. 71, 137 (1993)] that taking many non-adiabatic scatterers periodically in a quantum channel, the quantization can be recovered. We study this conductance quantization of a periodic system in the presence of a strong defect. A periodic arrangement of double-stubs give remarkable quantization of conductance. A periodic arrangement of double-constrictions also gives a very good quantization only when the separation between the constrictions is small. We conclude that conductance quantization of a periodically modulated channel is robust.

cond-mat.mes-hall↗

arXiv1997

Phase and periodicity of Aharonov-Bohm oscillations: effect of channel mixing

We take a negative delta function impurity in one arm of a quasi one dimensional Aharonov-Bohm ring and demonstrate abrupt phase changes across a quasi bound state of the negative delta function potential. We give a new mechanism for conductance oscillations with the strength of the negative delta potential. We also show that coupling to evanescent modes can result in a $hc/2e$ flux periodicity at certain energies. These observations were made in a recent ingenious experiment by D. Mailly et al [1].

cond-mat.mes-hall↗

arXiv1996

Nature of eigenstates in a mesoscopic ring coupled to a side branch

We formalize the parity effect of a multichannel ring coupled to a side branch using the tight binding model. We find that the tight binding model gives slightly different result from the continuum model. We also show that some states of the system can be nonmagnetic. In the multichannel ring coupled to a side branch, the persistent current in the ring does not change sign, over gigantic fluctuations in N (the number of particles in the ring).

cond-mat↗

arXiv1996

Phase of Aharonov-Bohm oscillations in conductance of mesoscopic systems

Motivated by a recent experiment we analyze in detail the phase of Aharonov-Bohm oscillations across a 1D ring with a stub coupled to one of its arms, in the presence of a magnetic flux. We demonstrate that there are two kinds of conductance extremas. One class of them are fixed at particular flux values and can only change abruptly from a maxima to a minima as incident energy is varied. We show a different mechanism for such abrupt phase change in conductance oscillation. We demonstrate that these extremas can exhibit $``$phase locking". However, the second kind of extremas can shift continuously as the incident energy is varied.

cond-mat↗

arXiv1996

Aharonov-Bohm effect in the presence of evanescent modes

It is known that differential magnetoconductance of a normal metal loop connected to reservoirs by ideal wires is always negative when an electron travels as an evanescent modes in the loop. This is in contrast to the fact that the magnetoconductance for propagating modes is very sensitive to small changes in geometric details and the Fermi energy and moreover it can be positive as well as negative. Here we explore the role of impurities in the leads in determining the magnetoconductance of the loop. We find that the change in magnetoconductance is negative and can be made large provided the impurities do not create resonant states in the systems. This theoretical finding may play an useful role in quantum switch operations.

cond-mat↗

arXiv1995

Revisiting Thouless conductance formula

It was shown using perturbation theory[1] that Thouless energy Ec for a quantum system scales linearly with the conductance of the system. We derive in an alternate way in 1-D that Ec scales with the conductance in a very different way. We physically show the difference between our approach and that of ref. 1 to expect our results to hold in higher dimensions also. We verify our results with exact numerical calculations.

cond-mat↗

arXiv1995

Effect of impurity on current magnification property of mesoscopic open rings

We have considered an open system consisting of a metallic ring coupled to two electron reservoirs. We have recently shown that in the presence of a transport current, circulating currents can flow in such a ring even in the absence of magnetic field. This is related to the current magnification effect in the ring. In our present work we have studied the effect of impurity on the current magnification. We find that the presence of impurity can enhance the current magnification in the loop significantly and thus lead to large circulating currents in certain range of Fermi energies. This is in contrast to the known fact that impurities can only decrease the persistent currents in a closed ring in the presence of magnetic flux.

cond-mat↗

arXiv1995

Topological effects on magnitude of persistent current in a ring

We show that defects in 1D rings decrease the conductance (when the ring is opened up) many more times than it decreases the persistent currents. This means that the states in such a 1D ring are very sensitive to twisting of boundary condition but conductance of the system is small. In 1D the electron effectively sees a periodic potential and escapes localization. This does not happen in higher dimensions. This helps us to understand the simulations of G. Kirczenow[13] and also suggests that a rough boundary in a 3D ring that provide effectively a strong 1D potential to the electron will have this type of different effects on conductance and persistent currents.

cond-mat↗

arXiv1995

How general is Legett's conjecture for a mesoscopic ring?

It has been shown[19] that in a loop of length u to which a single stub of length v is attached (fig. 1 in ref. 19), the parity effect is completely destroyed when v/u$>2$. It was also shown that such minute topological defects (v/u$<<1$) act as singular perturbations. However ref. 19 studies the effect of a single topological defect and says that for v/u$<<1$ parity effect is not violated in the ring. In this paper we show that topological defects of the type v/u$<<1$ can also violate the parity effect depending on the exact value of v/u and the parity effect is significantly destroyed if we have many such geometric scatterers. This paper brings out the physical reasons for the destruction of parity effect. We show that the generic feature of topological defects as this is that they can produce discontinuous phase change (due to change in energy) of the electron wavefunction in the ring for special value of v/u and then Legett's cojecture breaks down. This may have implications on the experimental observations.

cond-mat↗

arXiv1995

CURRENT MAGNIFICATION AND CIRCULATING CURRENTS IN MESOSCOPIC RINGS

We show that several novel effects related to persistent currents can arise in open systems, which have no analogue in closed or isolated systems. We have considered a system of a metallic ring coupled to two electron reservoirs. We show that in the presence of a transport current, persistent currents can flow in a ring even in the absence of magnetic field. This is related to the current magnification effect in the ring. In the presence of magnetic field we show that the amplitude of persistent currents is sensitive to the direction of current flow from one reservoir to another. Finally we briefly discuss the persistent currents arising due to two nonclassical effects namely, Aharonov-Bohm effect and quantum tunneling.

cond-mat↗

arXiv1995

Persistent Currents in the Presence of a Transport Current

We have considered a system of a metallic ring coupled to two electron reservoirs. We show that in the presence of a transport current, the persistent current can flow in a ring, even in the absence of magnetic field. This is purely a quantum effect and is related to the current magnification in the loop. These persistent currents can be observed if one tunes the Fermi energy near the antiresonances of the total transmission coefficient or the two port conductance.

cond-mat↗

arXiv1994

Quantum Waveguide Transport in Serial Stub and Loop Structures

We have studied the quantum transmission properties of serial stub and loop structures. Throughout we have considered free electron networks and the scattering arises solely due to the geometric nature of the problem. The band formation in these geometric structures is analyzed and compared with the conventional periodic potential scatterers. Some essential differences are pointed out. We show that a single defect in an otherwise periodic structure modifies band properties non trivially. By a proper choice of a single defect one can produce positive energy bound states in continuum in the sense of von Neumann and Wigner. We also discuss some magnetic properties of loop structures in the presence of Aharonov-Bohm flux.

cond-mat↗