arXiv subjects

A. Jagannathan

Publications and source records attributed to A. Jagannathan.

16 recordsLinked to original sources

arXiv2018

Probing Energy-Dependent Feshbach Resonances by Optical Control

Optical control enables new high resolution probes of narrow collisional (Feshbach) resonances, which are strongly dependent on the relative momentum of colliding atom pairs, and important for simulating neutron matter with ultracold atomic gases. We demonstrate a two-field optical vernier, which expands kHz (mG) magnetic field detunings near a narrow resonance into MHz optical field detunings, enabling precise control and characterization of the momentum-dependent scattering amplitude. Two-photon loss spectra are measured for the narrow resonance in $^6$Li, revealing rich structure in very good agreement with our theoretical model. However, anomalous frequency shifts between the measured and predicted two-photon spectra are not yet explained.

cond-mat.quant-gas↗

arXiv2018

Designer Spatial Control of Interactions in Ultracold Gases

Designer optical control of interactions in ultracold atomic gases has wide application, from creating new quantum phases to modeling the physics of black holes. We demonstrate spatial control of interactions in a two-component cloud of $^6$Li fermions, using electromagnetically induced transparency (EIT) to create a "sandwich" of resonantly and weakly interacting regions. Interaction designs are imprinted on the trapped cloud by two laser beams and manipulated with just MHz changes in the frequency of one beam. We employ radio-frequency spectroscopy to measure the imprinted 1D spatial profiles of the local mean-field interactions and to demonstrate that the tuning range of the scattering length is the same for both optical and magnetic control. All of the data are in excellent agreement with our continuum-dressed state theoretical model of optical control, which includes both the spatial and momentum dependence of the interactions.

cond-mat.quant-gas↗

arXiv2015

Two-field optical methods to control magnetic Feshbach resonances

Using an optically-trapped mixture of the two lowest hyperfine states of a $^6$Li Fermi gas, we observe two-field optical tuning of the narrow Feshbach resonance by up to 3 G and an increase in spontaneous lifetime near the broad resonance from $0.5$ ms to $0.4$ s. We present a new model of light-induced loss spectra, employing continuum-dressed basis states, that agrees in shape and magnitude with measurements for both broad and narrow resonances.

cond-mat.quant-gas↗

arXiv2012

Interplay between edge states and simple bulk defects in graphene nanoribbons

We study the interplay between the edge states and a single impurity in a zigzag graphene nanoribbon. We use tight-binding exact diagonalization techniques, as well as density functional theory calculations to obtain the eigenvalue spectrum, the eigenfunctions, as well the dependence of the local density of states (LDOS) on energy and position. We note that roughly half of the unperturbed eigenstates in the spectrum of the finite-size ribbon hybridize with the impurity state, and the corresponding eigenvalues are shifted with respect to their unperturbed values. The maximum shift and hybridization occur for a state whose energy is inverse proportional to the impurity potential; this energy is that of the impurity peak in the DOS spectrum. We find that the interference between the impurity and the edge gives rise to peculiar modifications of the LDOS of the nanoribbon, in particular to oscillations of the edge LDOS. These effects depend on the size of the system, and decay with the distance between the edge and the impurity.

cond-mat.mes-hall↗

arXiv2010

Novel Properties of Frustrated Low Dimensional Magnets with Pentagonal Symmetry

In the context of magnetism, frustration arises when a group of spins cannot find a configuration that minimizes all of their pairwise interactions simultaneously. We consider the effects of the geometric frustration that arises in a structure having pentagonal loops. Such five-fold loops can be expected to occur naturally in quasicrystals, as seen for example in a number of experimental studies of surfaces of icosahedral alloys. Our model considers classical vector spins placed on vertices of a subtiling of the two dimensional Penrose tiling, and interacting with nearest neighbors via antiferromagnetic bonds. We give a set of recursion relations for this system, which consists of an infinite set of embedded clusters with sizes that increase as a power of the golden mean. The magnetic ground states of this fractal system are studied analytically, and by Monte Carlo simulation.

cond-mat.stat-mech↗

arXiv2010

Weakening of antiferromagnetism due to frustration on inhomogeneous 2D lattices

This paper discusses the ground state of quantum spins interacting via Heisenberg antiferromagnetic exchange, in two dimensional lattices having several different local environments. In unfrustrated lattices, when a Néel type order exists, quantum fluctuations lead to an inhomogeneous order parameter. I consider the effects of adding frustration to such a system by including next nearest neighbor antiferromagnetic interactions. Within linear spin wave theory, it is found that as the frustration parameter increases, the local order parameters decrease at a rate that depends on the type of sites involved in the frustrated loops. The examples of a decorated square lattice system, and a quasiperiodic tiling are considered.

cond-mat.str-el↗

arXiv2006

Energy levels and their correlations in quasicrystals

Quasicrystals can be considered, from the point of view of their electronic properties, as being intermediate between metals and insulators. For example, experiments show that quasicrystalline alloys such as AlCuFe or AlPdMn have conductivities far smaller than those of the metals that these alloys are composed from. Wave functions in a quasicrystal are typically intermediate in character between the extended states of a crystal and the exponentially localized states in the insulating phase, and this is also reflected in the energy spectrum and the density of states. In the theoretical studies we consider in this review, the quasicrystals are described by a pure hopping tight binding model on simple tilings. We focus on spectral properties, which we compare with those of other complex systems, in particular, the Anderson model of a disordered metal.

cond-mat.str-el↗

arXiv2006

Inhomogeneous quantum antiferromagnetism on periodic lattices

We study quantum antiferromagnets on two-dimensional bipartite lattices. We focus on local variations in the properties of the ordered phase which arise due to the presence of inequivalent sites or bonds in the lattice structure, using linear spin wave theory and quantum Monte Carlo methods. Our primary finding is that sites with a high coordination tend to have a low ordered moment, at odds with the simple intuition of high coordination favoring more robust Neel ordering. The lattices considered are the dice lattice, which is dual to the kagome, the CaVO lattice, an Archimedean lattice with two inequivalent bonds, and the crown lattice, a tiling of squares and rhombi with a greater variety of local environments. We present results for the onsite magnetizations and local bond expectation values for the spin-1/2 Heisenberg model on these lattices, and discuss the exactly soluble model of a Heisenberg star, which provides a simple analytical framework for understanding our lattice studies.

cond-mat.str-el↗

arXiv2004

Ground state of a two dimensional quasiperiodic antiferromagnet

We consider the spin 1/2 Heisenberg antiferromagnet on a two dimensional quasiperiodic tiling. The T=0 state is long range ordered, with an inhomogeneous distribution of the staggered moment. An approximate real space renormalization scheme first developed for the square lattice is generalized and used to obtain information on the ground state energy as well as the distribution of local correlations in the quasicrystal.

cond-mat.str-el↗

arXiv2003

Quantum Spins and Quasiperiodicity: a real space renormalization group approach

We study the antiferromagnetic spin-1/2 Heisenberg model on a two-dimensional bipartite quasiperiodic structure, the octagonal tiling -- the aperiodic equivalent of the square lattice for periodic systems. An approximate block spin renormalization scheme is described for this problem. The ground state energy and local staggered magnetizations for this system are calculated, and compared with the results of a recent Quantum Monte Carlo calculation for the tiling. It is conjectured that the ground state energy is exactly equal to that of the quantum antiferromagnet on the square lattice.

cond-mat.str-el↗

arXiv2001

Less singular quasicrystals: life in low codimensions

The Rauzy tilings were proposed recently in a generalisation of the Fibonacci chain by Vidal and Mosseri. These tilings have a particularly simple theoretical description, making them appealing candidates for analytical solutions for electronic properties. We show by a numerical study of statistical properties of the tight-binding spectra that these tilings fall in an intermediate category between the crystal and the quasicrystal, i.e. in a class of almost integrable models.

cond-mat.dis-nn↗

arXiv1999

Self-similarity under inflation and level statistics: a study in two dimensions

Energy level spacing statistics are discussed for a two dimensional quasiperiodic tiling. The property of self-similarity under inflation is used to write a recursion relation for the level spacing distributions defined on square approximants to the perfect quasiperiodic structure. New distribution functions are defined and determined by a combination of numerical and analytical calculations.

cond-mat.mes-hall↗

arXiv1996

Magnetic states induced by electron-electron interactions in a plane quasiperiodic tiling

We consider the Hubbard model for electrons in a two-dimensional quasiperiodic tiling using the Hartree--Fock approximation. Numerical solutions are obtained for the first three square approximants of the perfect octagonal tiling. At half-filling, the magnetic state is antiferromagnetic. We calculate the distributions of local magnetizations and their dependence on the local environments as U (Hubbard repulsion strength) is varied. The inflation symmetry of quasicrystals results in a corresponding inflation symmetry of the magnetic configurations when passing from one tiling to the next in the progression towards the infinite quasicrystal.

cond-mat↗

arXiv1995

Model for the thermal conductivity of a quasicrystalline alloy

We present a simple model for the observed temperature dependence of the thermal conductivity of AlPdMn (Chernikov et al, Phys.Rev.B vol.51, (1995),153). We account for the low temperature data, upto and including the plateau of the thermal conductivity, and discuss the similarities and the differences between the quasicrystal and amorphous solids.

cond-mat↗

arXiv1995

Analytical results for Scaling Properties of the Spectrum of the Fibonacci Chain

We solve the approximate renormalisation group found by Qiu Niu and Franco Nori(Phys. Rev. Lett. 57 2057(1986)) for a tight-binding hamiltonian on the Fibonacci chain. This enables us to characterize analytically as completely as possible the spectral properties of this model, such as the spectral measure, the bandwidth distribution, the Lebesgue measure exponent, the Hausdorff dimension, the multifractal scaling, the gaps distribution as well as the long time return probability. Our results, qualitatively and quantitatively complete and unify previous works on similar models.

cond-mat↗

arXiv1993

Density of states and magnetic susceptibilities on the octagonal tiling

We study electronic properties as a function of the six types of local environments found in the octagonal tiling. The density of states has six characteristic forms, although the detailed structure differs from site to site since no two sites are equivalent in a quasiperiodic tiling. We present the site-dependent magnetic susceptibility of electrons on this tiling, which also has six characteristic dependences. We show the existence of a non-local spin susceptibility, which decays with the square of the distance between sites and is the quasiperiodic version of Ruderman-Kittel oscillations. These results are obtained for a tight-binding Hamiltonian with pure hopping.Finally, we investigate the formation of local magnetic moments when electron-electron interactions are included.

cond-mat↗