arXiv subjects

Sumathi Rao

Publications and source records attributed to Sumathi Rao.

At least 73 records · Page 4Linked to original sources

arXiv2003

Quantised charge pumping through multiple quantum dots

We study electron pumping through a system of barriers, whose heights are deformed adiabatically. We derive a simple formula for the pumped charge $Q$ in terms of the total reflection and transmission amplitudes and phases. The pumped charge increases with the number of barriers ($n_b$) and shows an interesting step-like behaviour, with the steps appearing at integer values of $Q$. The pumped charge also tends towards quantisation with the increase of the amplitude of the time-varying potential. The value of the quantised pumped charge is shown to be correlated to the discontinuity of the reflection phase.

cond-mat.mes-hall↗

arXiv2003

Effective action and interaction energy of coupled quantum dots

We obtain the effective action of tunnel-coupled quantum dots, by modeling the system as a Luttinger liquid with multiple barriers. For a double dot system, we find that the resonance conditions for perfect conductance form a hexagon in the plane of the two gate voltages controlling the density of electrons in each dot. We also explicitly obtain the functional dependence of the interaction energy and peak-splitting on the gate voltage controlling tunneling between the dots and their charging energies. Our results are in good agreement with recent experimental results, from which we obtain the Luttinger interaction parameter $K=0.74$.

cond-mat.mes-hall↗

arXiv2002

Junction of several weakly interacting quantum wires: a renormalization group study

We study the conductance of three or more semi-infinite wires which meet at a junction. The electrons in the wires are taken to interact weakly with each other through a short-range density-density interaction, and they encounter a general scattering matrix at the junction. We derive the renormalization group equations satisfied by the S-matrix, and we identify its fixed points and their stabilities. The conductance between any pair of wires is then studied as a function of physical parameters such as temperature. We discuss the possibility of observing the effects of junctions in present day experiments, such as the four-terminal conductance of a quantum wire and crossed quantum wires.

cond-mat.str-el↗

arXiv2001

An introduction to bosonization and aome of its applications

We discuss the technique of bosonization for studying systems of interacting fermions in one dimension. After briefly reviewing the low-energy properties of Fermi and Luttinger liquids, we present some of the relations between bosonic and fermionic operators in one dimension. We use these relations to calculate the correlation functions and the renormalization group properties of various operators for a system of spinless fermions. We then apply the methods of bosonization to study the Heisenberg antiferromagnetic spin 1/2 chain, the Hubbard model in one dimension, and transport in clean quantum wires and in the presence of isolated impurities.

cond-mat.str-el↗

arXiv2001

Transport in quantum wires

With a brief introduction to one-dimensional channels and conductance quantisation in mesoscopic systems, we discuss some recent experimental puzzles in these systems, which include reduction of quantised conductances and an interesting {\it odd-even} effect in the presence of an in-plane magnetic field. We then discuss a recent non-homogeneous Luttinger liquid model proposed by us, which addresses and gives an explanation for the reduced conductances and the {\it odd-even} effect. We end with a brief summary and discussion of future projects.

cond-mat.mes-hall↗

arXiv2001

An Anyon Primer

In this set of lectures, we give a pedagogical introduction to the subject of anyons. We discuss 1) basic concepts in anyon physics, 2) quantum mechanics of two anyon systems, 3) statistical mechanics of many anyon systems, 4) mean field approach to many anyon systems and anyon superconductivity, 5) anyons in field theory and 6) anyons in the Fractional Quantum Hall Effect (FQHE). (Based on lectures delivered at the VII SERC school in High Energy Physics at the Physical Research Laboratory, Ahmedabad, January 1992 and at the I SERC school in Statistical Mechanics at Puri, February 1994.)

hep-th↗

arXiv2001

Conductance through contact barriers of a finite length quantum wire

We use the technique of bosonization to understand a variety of recent experimental results on the conductivity of a quantum wire. The quantum wire is taken to be a finite-length Luttinger liquid connected on two sides to semi-infinite Fermi liquids through contacts. The contacts are modeled as (short) Luttinger liquids bounded by localized one-body potentials. We use effective actions and the renormalization group to study the effects of electronic interactions within the wire, the length of the wire, finite temperature and a magnetic field on the conductivity. We explain the deviations of the conductivity away from 2Ne^2/h in wires which are not too short as arising from renormalization effects caused by the repulsive interactions. We also explain the universal conductance corrections observed in different channels at higher temperatures. We study the effects of an external magnetic field on electronic transport through this system and explain why odd and even spin split bands show different renormalizations from the universal conductance values. We discuss the case of resonant transmission and of the possibility of producing a spin-valve which only allows electrons of one value of the spin to go through. We compare our results for the conductance corrections with experimental observations. We also propose an experimental test of our model of the contact regions.

cond-mat.str-el↗

arXiv2000

Transport through quasi-ballistic quantum wires: the role of contacts

We model one-dimensional transport through each open channel of a quantum wire by a Luttinger liquid with three different interaction parameters for the leads, the contact regions and the wire, and with two barriers at the contacts. We show that this model explains several features of recent experiments, such as the flat conductance plateaux observed even at finite temperatures and for different lengths, and universal conductance corrections in different channels. We discuss the possibility of seeing resonance-like structures of a fully open channel at very low temperatures.

cond-mat.str-el↗

arXiv1999

Transport in a class of exactly solvable models of interacting fermions

We study transport in a class of exactly solvable models of interacting fermions in one dimension. We contrast these models with models of non-interacting fermions in an Aharanov-Bohm ring to which they are superficially similar. We introduce magnetic and non-magnetic impurities at a site, through either a weak $δ$-function potential or through a weak link. Using a renormalisation group analysis, we show that the strength of the nonmagnetic impurity is not affected by the interaction, whereas the magnetic impurity cuts the wire at the impurity site.

cond-mat.str-el↗

arXiv1999

Results from bosonisation for resonant tunneling through a quantum dot in an Aharanov-Bohm ring

We study coherent charge tunneling through a one-dimensional interacting ring with a one-dimensional quantum dot embedded in one of its arms through bosonisation. The symmetries of the effective action explain many of the features such as phase change between resonances, in-phase successive resonances and phase-locking, which have been observed in experiments of coherent transport in mesoscopic rings, with a quantum dot. We also predict changes in the behaviour of the tunneling conductance in the presence of an Aharanov-Bohm flux through the ring. We argue that these results hold true in general for any dot.

cond-mat.mes-hall↗

arXiv1999

Transport in Luttinger Liquids

We give a brief introduction to Luttinger liquids and to the phenomena of electronic transport or conductance in quantum wires. We explain why the subject of transport in Luttinger liquids is relevant and fascinating and review some important results on tunneling through barriers in a one-dimensional quantum wire and the phenomena of persistent currents in mesoscopic rings. We give a brief description of our own work on transport through doubly-crossed Luttinger liquids and transport in the Schulz-Shastry exactly solvable Luttinger-like model.

cond-mat.str-el↗

arXiv1999

The Fractional Quantum Hall Effect

We give a brief introduction to the phenomenon of the Fractional Quantum Hall effect, whose discovery was awarded the Nobel prize in 1998. We also explain the composite fermion picture which describes the fractional quantum Hall effect as the integer quantum Hall effect of composite fermions.

cond-mat.mes-hall↗

arXiv1998

Transport in Double-Crossed Luttinger Liquids

We study transport through two Luttinger liquids (one-dimensional electrons interacting through a Coulomb repulsion in a metal) coupled together at {\it two} points. External voltage biases are incorporated through boundary conditions. We include density-density couplings as well as single-particle hops at the contacts. For weak repulsive interactions, transport through the wires remains undisturbed by the inter-wire couplings, which renormalise to zero. For strong repulsive interactions, the inter-wire couplings become strong. For symmetric barriers and no external voltage bias, a single gate voltage is sufficient to tune for resonance transmission in both wires. However, for asymmetric couplings or for finite external biases, the system is insulating.

cond-mat.str-el↗

arXiv1998

Exactly Solvable Fermionic N-chain Models

Motivated by the problem of N coupled Hubbard chains, we investigate a generalisation of the Schulz-Shastry model containing two species of one-dimensional fermions interacting via a gauge field that depends on the positions of all the particles of the other species. The exact many body ground state of the model can be easily obtained through a unitary transformation of the model. The correlation functions are Luttinger-like - i.e., they decay through power laws with non-integer exponents. Through the interaction dependent correlation functions of the two-particle operators, we identify the relevant perturbations and hence, possible instabilities.

cond-mat.stat-mech↗

arXiv1997

Exact Analytic Results for Composite Fermions in a Rajaraman-Sondhi like formulation

We obtain the exact spectrum and the unique ground state of two composite fermions (in a Rajaraman - Sondhi like formulation) in an external magnetic field $B$. We show that the energy eigenvalues decrease with increasing angular momentum, thus making it energetically favourable for composite fermions to stay apart. Generalising this result to a gas of composite fermions, we provide an energetic justification of the Laughlin and Jain wave-functions.

cond-mat.mes-hall↗

arXiv1997

Fractional Quantum Hall Effect States as Exact Ground States

We construct many particle Hamiltonians for which the Laughlin and Jain wavefunctions are exact ground states. The Hamiltonians involve fermions in a magnetic field and with inter-particle interactions. For the Laughlin wave-functions,the interactions involve two- and three-body correlations similar to the Chern-Simons interactions, whereas for the projected Jain wave-functions, N-body interactions (which cannot be explicitly written down in general) are involved.

cond-mat.mes-hall↗

arXiv1997

Exactly Solvable Models in Arbitrary Dimensions

We construct a new class of quasi-exactly solvable many-body Hamiltonians in arbitrary dimensions, whose ground states can have any correlations we choose. Some of the known correlations in one dimension and some recent novel correlations in two and higher dimensions are reproduced as special cases. As specific interesting examples, we also write down some new models in two and higher dimensions with novel correlations.

hep-th↗

arXiv1996

Semiclassical and field theoretic studies of Heisenberg antiferromagnetic chains with frustration and dimerization

The Heisenberg antiferromagnetic spin chain with both dimerization and frustration is studied. The classical ground state has three phases (a Neel phase, a spiral phase and a colinear phase), around which a planar spin-wave analysis is performed. In each phase, we discuss a non-linear sigma model field theory describing the low energy excitations. A renormalization group analysis of the SO(3) matrix-valued field theory of the spiral phase leads to the conclusion that the theory becomes $SO(3) \times SO(3)$ and Lorentz invariant at long distances. This theory is analytically known to have a massive spin-1/2 excitation. We also show that $Z_2 ~$ solitons in the field theory lead to a double degeneracy in the spectrum for half-integer spins.

cond-mat↗