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V. Tripathi

Publications and source records attributed to V. Tripathi.

At least 37 records · Page 2Linked to original sources
arXiv2008

Nuclear magnetic resonance probes for the Kondo scenario for the 0.7 feature in semiconductor quantum point contact devices

We propose a probe based on nuclear relaxation and Knight shift measurements for the Kondo scenario for the "0.7 feature" in semiconductor quantum point contact (QPC) devices. We show that the presence of a bound electron in the QPC would lead to a much higher rate of nuclear relaxation compared to nuclear relaxation through exchange of spin with conduction electrons. Furthermore, we show that the temperature dependence of this nuclear relaxation is very non-monotonic as opposed to the linear-T relaxation from coupling with conduction electrons. We present a qualitative analysis for the additional relaxation due to nuclear spin diffusion (NSD) and study the extent to which NSD affects the range of validity of our method. The conclusion is that nuclear relaxation, in combination with Knight shift measurements, can be used to verify whether the 0.7 feature is indeed due to the presence of a bound electron in the QPC.

cond-mat.mes-hall↗
arXiv2008

Dynamic nuclear polarisation in biased quantum wires with spin-orbit interaction

We propose a new method for dynamic nuclear polarisation in a quasi one-dimensional quantum wire utilising the spin-orbit interaction, the hyperfine interaction, and a finite source-drain potential difference. In contrast with current methods, our scheme does not rely on external magnetic or optical sources which makes independent control of closely placed devices much more feasible. Using this method, a significant polarisation of a few per cent is possible in currently available InAs wires which may be detected by conductance measurements. This may prove useful for nuclear-magnetic-resonance studies in nanoscale systems as well as in spin-based devices where external magnetic and optical sources will not be suitable.

cond-mat.mes-hall↗
arXiv2007

Theory of NMR in semiconductor quantum point contact devices

We describe how a local non-equilibrium nuclear polarisation can be generated and detected by electrical means in a semiconductor quantum point contact device. We show that measurements of the nuclear spin relaxation rate will provide clear signatures of the interaction mechanism underlying the "0.7" conductance anomaly. Our analysis illustrates how nuclear magnetic resonance methods, which are used extensively to study strongly-correlated electron phases in bulk materials, can be made to play a similarly important role in nanoscale devices.

cond-mat.mes-hall↗
arXiv2007

Magnetic field induced Coulomb blockade in small disordered delta-doped heterostructures

At low densities, electrons confined to two dimensions in a delta-doped heterostructure can arrange themselves into self-consistent droplets due to disorder and screening effects. We use this observation to show that at low temperatures, there should be resistance oscillations in low density two dimensional electron gases as a function of the gate voltage, that are greatly enhanced in a magnetic field. These oscillations are intrinsic to small samples and give way to variable range hopping resistivity at low temperatures in larger samples. We place our analysis in the context of recent experiments where similar physical effects have been discussed from the point of view of a Wigner crystal or charge density wave picture.

cond-mat.str-el↗
arXiv2007

Sleuthing Hidden Order

Physicists have long debated whether the hidden order in URu2Si2 is itinerant or localized, and it remains inaccessible to direct external probes. Recent observation of an overdamped collective mode in this material (C. Weibe et al, Nature Physics 3, 96-100 (2007)), appears to resolve this outstanding issue.

cond-mat.str-el↗
arXiv2006

Magnetotransport in disordered delta-doped heterostructures

We discuss theoretically how electrons confined to two dimensions in a delta-doped heterostructure can arrange themselves in a droplet-like spatial distribution due to disorder and screening effects when their density is low. We apply this droplet picture to magnetotransport and derive the expected dependence on electron density of several quantities relevant to this transport, in the regimes of weak and moderate magnetic fields. We find good qualitative and quantitative agreement between our calculations and recent experiments on delta-doped heterostructures.

cond-mat.str-el↗
arXiv2006

Optical conductivity of a granular metal at not very low temperatures

We study the finite-temperature optical conductivity, sigma(omega,T), of a granular metal using a simple model consisting of a array of spherical metallic grains. It is necessary to include quantum tunneling and Coulomb blockade effects to obtain the correct temperature dependence of sigma(omega, T), and to consider polarization oscillations to obtain the correct frequency dependence. We have therefore generalized the Ambegaokar-Eckern-Schoen (AES) model for granular metals to obtain an effective field theory incorporating the polarization fluctuations of the individual metallic grains. In contrast to the DC conductivity, which is determined by inter-grain charge transfer and obeys an Arrhenius law at low temperature, the AC conductivity is dominated by a resonance peak for intra-grain polarization oscillations, which has a power-law tail at low frequencies. More importantly, although the resonance frequency agrees with the classical prediction, the resonance width depends on intergrain quantum tunneling and Coulomb blockade parameters, in addition to the classical Drude relaxation within the grain. This additional damping is due to inelastic cotunneling of polarization fluctuations to neighbouring grains and it qualitatively differs from the DC conductivity in its temperature dependence quite unlike the expectation from Drude theory.

cond-mat.mes-hall↗
arXiv2006

Influence of projectile - breakup threshold on complete fusion

Complete fusion excitations for 11,10B+159Tb have been measured at energies around the respective Coulomb barriers, and the existing complete fusion measurements for 7Li+159Tb have been extended to higher energies. The measurements show significant reduction of complete fusion cross sections at above-barrier energies for both the reactions, 10B+159Tb and 7Li+159Tb, when compared to those for 11B+159Tb. The comparison shows that the extent of suppression of complete fusion cross sections is correlated with the -separation energies of the projectiles. Also, the two reactions, 10B+159Tb and 7Li+159Tb were found to produce incomplete fusion products at energies near the respective Coulomb barriers, with the - particle emitting channel being the favoured incomplete fusion process in both the cases.

nucl-ex↗
arXiv2005

Thermal transport in a granular metal array

We obtain the Kubo formula for the electronic thermal conductivity kappa(T) of a granular metal array at low temperatures for the Ambegaokar-Eckern-Schoen (AES) model and study the kinetic and potential contributions in the diamagnetic (local) and paramagnetic (current-current) terms. For small values of dimensionless intergrain tunneling conductance, g << 1, we show that inelastic cotunneling processes contribute to thermal conductivity due to non-cancellation of the diamagnetic and paramagnetic terms, unlike electrical conductivity. We find that the electrical conductivity obeys the Arrhenius law, sigma(T) ~ ge^{-E_c/T}, however kappa(T) decreases only algebraically, kappa(T) \~ g^2 T^3/E_c^2. At large values of intergrain coupling, g >> 1, we find it plausible that the Wiedemann-Franz law weakly deviates from the free-electron theory due to Coulomb effects.

cond-mat.mes-hall↗
arXiv2005

Itinerancy and Hidden Order in $URu_2Si_2$

We argue that key characteristics of the enigmatic transition at $T_0= 17.5K $ in $URu_2Si_2$ indicate that the hidden order is a density wave formed within a band of composite quasiparticles, whose detailed structure is determined by local physics. We expand on our proposal (with J.A. Mydosh) of the hidden order as incommnesurate orbital antiferromagnetism and present experimental predictions to test our ideas. We then turn towards a microscopic description of orbital antiferromagnetism, exploring possible particle-hole pairings within the context of a simple one-band model. We end with a discussion of recent high-field and thermal transport experiment, and discuss their implications for the nature of the hidden order.

cond-mat.str-el↗
arXiv2005

Effective charging energy for a regular granular metal array

We study the Ambegaokar-Eckern-Schön (AES) model for a regular array of metallic grains coupled by tunnel junctions of conductance $g$ and calculate both paramagnetic and diamagnetic terms in the Kubo formula for the conductivity. We find analytically, and confirm by numerical path integral Monte Carlo methods, that for $0<g<4$ the conductivity obeys an Arrhenius law $σ(T)\sim\exp[-E^{*}(g)/T]$ with an effective charging energy $E^{*} (g)$ when the temperature is sufficiently low, due to a subtle cancellation between $T^2$ inelastic-cotunneling contributions in the paramagnetic and diamagnetic terms. We present numerical results for the effective charging energy and compare the results with recent theoretical analyses. We discuss the different ways in which the experimentally observed $σ(T)\sim\exp[-\sqrt{T_{0}/T}]$ law could be attributed to disorder.

cond-mat.mes-hall↗
arXiv2004

$p$-$sd$ shell gap reduction in neutron-rich systems and cross-shell excitations in $^{20}$O

Excited states in $^{20}$O were populated in the reaction $^{10}$Be($^{14}$C,$α$) at Florida State University. Charged particles were detected with a particle telescope consisting of 4 annularly segmented Si surface barrier detectors and $γ$ radiation was detected with the FSU $γ$ detector array. Five new states were observed below 6 MeV from the $α$-$γ$ and $α$-$γ$-$γ$ coincidence data. Shell model calculations suggest that most of the newly observed states are core-excited 1p-1h excitations across the $N = Z = 8$ shell gap. Comparisons between experimental data and calculations for the neutron-rich O and F isotopes imply a steady reduction of the $p$-$sd$ shell gap as neutrons are added.

nucl-ex↗
arXiv2004

Coulomb blockade and quantum tunnelling in the low-conductivity phase of granular metals

We study the effects of Coulomb interaction and inter-grain quantum tunnelling in an array of metallic grains using the phase-functional approach for temperatures $T$ well below the charging energy $E_{c}$ of individual grains yet large compared to the level spacing in the grains. When the inter-grain tunnelling conductance $g\gg1$, the conductivity $σ$ in $d$ dimensions decreases logarithmically with temperature ($σ/σ_{0}\sim1-\frac{1}{2πgd}\ln(gE_{c}/T)$), while for $g\to0$, the conductivity shows simple activated behaviour ($σ\sim \exp(-E_c/T)$). We show, for bare tunnelling conductance $g \gtrsim 1$, that the parameter $γ\equiv g(1-2/(gπ)\ln(gE_{c}/T))$ determines the competition between charging and tunnelling effects. At low enough temperatures in the regime $1\gtrsim γ\gg 1/\sqrt{βE_{c}}$, a charge is shared among a finite number $N=\sqrt{(E_{c}/T)/\ln(π/2γz)}$ of grains, and we find a soft activation behaviour of the conductivity, $σ\sim z^{-1}\exp(-2\sqrt{(E_{c}/T)\ln(π/2γz)})$, where $z$ is the effective coordination number of a grain.

cond-mat.mes-hall↗
arXiv2004

Magnetic droplets in a metal close to a ferromagnetic quantum critical point

Using analytical and path integral Monte Carlo methods, we study the susceptibility $χ_{dc}(T)$ of a spin-S impurity with XY rotational symmetry embedded in a metal. Close to a ferromagnetic quantum critical point, the impurity polarizes conduction electrons in its vicinity and forms a large magnetic droplet with moment M>>S. At not too low temperatures, the strongly damping paramagnon modes of the conduction electrons suppress large quantum fluctuations (or spin flips) of this droplet. We show that the susceptibility follows the law $χ_{dc}(T)=(M^{2}/T)[1-(πg)^{-1}\ln(gE_{0}/T)]$, where the parameter g>>1 describes the strong damping by conduction electrons, and E_0 is the bandwidth of paramagnon modes. At exponentially low temperatures T << T_{*} ~ E_{0}\exp(-πg/2) we show that spin flips cannot be ignored. In this regime we find that $χ_{dc}(T) \approx χ_{dc}(0) [1-(2/3)(T/T_{*})^2]$, where $χ_{dc}(0)\sim M^{2}/T_{*}$ is finite and exponentially large in g. We also discuss these effects in the context of the multi-channel Kondo impurity model.

cond-mat.str-el↗
arXiv2003

The Case for Phase Separation in $URu_2Si_2$

Motivated by experiment, we review the case for phase inhomogeneity in URu_2Si_2. In this scenario, the paramagnetic hidden order phase coexists with small distinct domains of antiferromagnetism whose volume fraction increases with pressure. The implications for the nature of the hidden order are discu ssed.

cond-mat.str-el↗
arXiv2002

Hidden Order in $URu_{2}Si_{2}$: The Need for a Dual Description

Motivated by experiment, we argue that the enigmatic hidden order in $URu_2Si_2$ demands a dual description that encompasses both its itinerant and its local aspects. A combination of symmetry considerations and observation allow us to rule out a number of possibilities. The two remaining scenarios, the quadrupolar charge density wave and the orbital antiferromagnet, are discussed and experiments are suggested to select between these proposals.

cond-mat.str-el↗
arXiv2002

Hidden Orbital Order in $URu_{2}Si_{2}$

When matter is cooled from high temperatures, collective instabilities develop amongst its constituent particles that lead to new kinds of order. An anomaly in the specific heat is a classic signature of this phenomenon. Usually the associated order is easily identified, but sometimes its nature remains elusive. The heavy fermion metal $URu_2Si_2$ is one such example, where the order responsible for the sharp specific heat anomaly at $T_0=17 K$ has remained unidentified despite more than seventeen years of effort. In $URu_{2}Si_{2}$, the coexistence of large electron-electron repulsion and antiferromagnetic fluctuations in $URu_2Si_2$ leads to an almost incompressible heavy electron fluid, where anisotropically paired quasiparticle states are energetically favored. In this paper we use these insights to develop a detailed proposal for the hidden order in $URu_2Si_2$. We show that incommensurate orbital antiferromagnetism, associated with circulating currents between the uranium ions, can account for the local fields and entropy loss observed at the $17 K$ transition; furthermore we make detailed predictions for neutron scattering measurements.

cond-mat.str-el↗