33 research outputs found
Theory of tunneling conductance of graphene normal metal-insulator-superconductor junctions
No full textWe calculate the tunneling conductance of a graphene normal metal-insulator-superconductor (NIS) junction with a barrier of thickness d and with an arbitrary voltage V0 applied across the barrier region. We demonstrate that the tunneling conductance of such a NIS junction is an oscillatory function of both d and V0. We also show that the periodicity and amplitude of such oscillations deviate from their universal values in the thin barrier limit as obtained in an earlier work [ S. Bhattacharjee and K. Sengupta Phys. Rev. Lett. 97 217001 (2006)] and become a function of the applied voltage V0. Our results reproduce the earlier results on tunneling conductance of such junctions in the thin [ S. Bhattacharjee and K. Sengupta Phys. Rev. Lett. 97 217001 (2006)] and zero [ C. W. J. Beenakker Phys. Rev. Lett. 97 067007 (2006)] barrier limits as special limiting cases. We discuss the experimental relevance of our results
Theory of tunneling conductance of graphene normal metal-insulator-superconductor junctions
No full textWe calculate the tunneling conductance of a graphene normal metal-insulator-superconductor (NIS) junction with a barrier of thickness d and with an arbitrary voltage applied across the barrier region. We demonstrate that the tunneling conductance of such a NIS junction is an oscillatory function of both d and . We also show that the periodicity and amplitude of such oscillations deviate from their universal values in the thin barrier limit as obtained in an earlier work [S. Bhattacharjee and K. Sengupta, Phys. Rev. Lett. 97, 217001 (2006)] and become a function of the applied voltage . Our results reproduce the earlier results on tunneling conductance of such junctions in the thin [S. Bhattacharjee and K. Sengupta, Phys. Rev. Lett. 97, 217001 (2006)] and zero [C. W. J. Beenakker, Phys. Rev. Lett. 97, 067007 (2006)] barrier limits as special limiting cases. We discuss the experimental relevance of our results
Quantum destruction of spiral order in two-dimensional frustrated magnets
No full textWe study the fate of spin-1/2 spiral-ordered two-dimensional quantum antiferromagnets that are disordered by quantum fluctuations. A crucial role is played by the topological point defects of the spiral phase, which are known to have a Z(2) character. Previous works established that a nontrivial quantum spin-liquid phase results when the spiral is disordered without proliferating the Z(2) vortices. Here, we show that when the spiral is disordered by proliferating and condensing these vortices, valence-bond solid ordering occurs due to quantum Berry phase effects. We develop a general theory for this latter phase transition and apply it to a lattice model. This transition potentially provides a new example of a Landau-forbidden deconfined quantum critical point
Some Unconventional Phases And Phase Transitions In Condensed Matter : Spin-Nematics, Spin-Liquids, Deconfined Critical Points And Graphene NIS Junctions
No full textCondensed matter physics provides us with an opportunity to explore a large variety of systems with diverse properties. Central to the understanding of these systems is a characterization of the nature of their ground states and low energy excitation. Often, such systems show various forms of emergent properties that are absent in the microscopic level. Identification of such emergent phases of condensed matter form an important avenue of research in the field. In this thesis example of such phases and their associated phase transitions have been studied.
The work presented here may be broadly divided into two themes: construction of the theoretical framework for understanding materials already studied experimentally, and, trying to provide new theoretical avenues which may be relevant for understanding future experiments. In these studies we shall explore some unconventional phases and phase transitions that may occur in condensed matter systems. A comprehensive understanding of the properties of such unconventional phases and phase transitions is important in the context of the large array of experimentally studied materials that regularly defy conventional wisdom in more than one way. The thesis consists of two distinct parts. In the first part we study three problems in frustrated magnets. The second part consists of studies of the tunnelling spectroscopy of metal-insulator-superconductor junctions in graphene.
Studies in frustrated magnets have opened up the possibility of existence of a whole range of phases beyond the already known magnetically ordered ones. Some of these new phases, like the spin nematic or the valence bond solid, display some other conventional order themselves. Others, like the much sort after spin liquid phases displays a whole new kind of order that cannot be captured through the celebrated Landau’s classification of phases on the basis of symmetry breaking and associated order parameters. The phase transitions in these systems are also equally interesting and lead to intriguing possibilities that demand new modes of analysis. In this part of the thesis we shall study the different properties of three magnets with spin-1/2, 1 and 3/2 respectively.
We start by providing an introduction to frustrated spin systems in Chapter [1]. The origin of antiferromagnetic interactions in Mott insulators is discussed and the concept of frustration of magnetic interaction is explained. We also point out the causes that may destroy magnetic order in spin systems, particularly the role of quantum fluctuations in presence or absence of magnetic frustration. This is followed with a brief outline of various magnetically ordered and disordered ground states with particular emphasis on the description of the later. We also give a brief outline of various properties of such phases and associated quantum phase transitions particularly noting the influences of quantum interferences encoded in the Berry phase terms. A brief description of the finite temperature properties is also provided. We end an outline of various experimentally relevant compounds that requires comprehensive understanding, some of which have been addressed in this thesis.
In Chapter [2] we study the properties of a spin-nematic state in context of the recently discovered spin-1 Mott insulator Nickel Gallium Sulphide (NiGa2S4). This isotropic triangular lattice compound shows no spin ordering till low temperatures. We propose that it may have a particular type of spin-nematic ground state and explain the experimentally observed properties of the compound on the basis of our proposal. Starting from a two band Hubbard model description, relevant for the compound, we derive the Bilinear Biquadratic spin Hamiltonian. We then show, within mean field theory, that this Hamiltonian describes a transition from the spiral state to a ferro-nematic state as a function of the ratio of bilinear and biquadratic couplings. We also study the possible effects of small pinning disorder andmagnetic field and suggest experiments that can possibly distinguish the proposed nematic state from others.
In Chapter [3] we explore the effects of the magneto-elastic coupling in the spin-3/2 B-site chromite spinel Cadmium Chromite (CdCr2O4). In this compound the spins form a pyrochlore lattice. Nearest neighbour spins interact antiferromagnetically. Due to frustration the system does not order at low temperatures and instead goes into a classical spin liquid state. Such a cooperative paramagnet is very susceptible to external perturbations which may relieve their frustration. In CdCr2O4, at lower temperatures the magnetic frustration is relieved by distorting the lattice through a first order magnetoelastic transition. Thus the compound presents a case where the relevant perturbation to the frustrated spin interactions is provided by spin-phonon coupling. An effect of such perturbations on a cooperative paramagnet is of general interest and all aspects of this are not understood presently. We take the initial step of characterizing the spin-phonon interaction in detail. Based on recent sound velocity experiments, we construct a microscopic theory for the sound velocity renormalization due to the spin-phonon coupling and explain the recent experimental data obtained by S. Zherlitsyn et al. using our theory we can explain the dependence of the sound velocity on temperature as well as magnetic field. We also construct a Landau theory to explain (qualitatively) the behaviour of sound velocity across the magneto-structural transition. Further, we discuss the effects due to the small Dzyaloshinskii-Moriya interaction that may be present in these compounds.
In Chapter [4] we study the possibility of a direct second order quantum phase transition from spiral to dimer phase in two dimensional antiferromagnets. Such transitions between phases with incompatible symmetries are forbidden within conventional Landau Ginzburg-Wilson paradigm of critical phenomena. Early works showed that when the spiral is destroyed by long wavelength fluctuations a fractionalized Z2 spin liquid is obtained. In this work we show an alternative way–the quantum destruction of the spiral magnet. We argue that, when the defects of the spiral phase proliferate and condense, their associated Berry phase automatically leads to dimerization. We apply our theory to study concrete lattice models where such transitions may be observed. This transition is an example of a Landau forbidden deconfined quantum phase transition. The proposed critical theory is naturally written in terms of fractional degrees of freedom which emerge right at the critical point. These fractional particles interact with each other through emergent gauge fields and are deconfined right at the critical point (but are confined in either of the two adjoining phases). We argue, based on existing results, that the monopoles of the gauge field are dangerously irrelevant right at the critical point rendering the later noncompact. The critical point is characterized by an emergent global U (1) conservation law that is absent in the microscopic model, a typical feature of a deconfined quantum critical point. The resultant field theory belongs to the class of anisotropic NCCP3 class which may be studied numerically in future to understand its critical properties.
In modern condensed matter physics the emergence of new and novel phases of matter have often been associated with the presence of strong correlations. Indeed, strongly correlated systems seem to harbour in them the potential to realize some of the most unconventional and exotic emergent phases of matter. However in graphene, which is a single layer of graphite, the emergence of novel properties, as present experiments suggest, is due to its unique band structure and not a fallout of intricate correlation effects. Band structure studies of graphene suggest that the material is a zero gap semiconductor with the low energy excitations resembling massless Dirac quasi-particles. The consequence of this is immediate and interesting. It has lead to the possibility of exploring the physics of relativistic fermions in two spatial dimensions and much of this has been studied with great vigour in the last five years.
In our studies, presented in Chapter [5], we explore one of the many consequence of this emergent Dirac structure of the low energy quasi-particles, namely the properties of metal-insulator-superconductor junctions of graphene. The twin effect of Klein tunneling of Dirac fermions (and associated transmission resonances) and Andreev reflection (both specular and retro) sets them aside from their conventional counterparts. The graphene normal metal-insulator-superconductor (NIS) junctions show strikingly different properties like oscillations in the sub-gap tunneling conductance as a function of both barrier strength and width. We make a detailed study of this for arbitrary barrier strengths and widths with and without Fermi-surface mismatch between the normal and the superconducting sides. The amplitude of these oscillations are maximum for aligned Fermi surface and vanishes for large Fermi surface mismatch. We provide an understanding for this unconventional behaviour of graphene NIS junctions. We also suggest experimental tests for our theory. Such experimental verification will reveal one more remarkable emergent property in a condensed matter system
Tunneling conductance of graphene NIS junctions
No full textWe show that, in contrast with conventional normal metal-insulator-superconductor (NIS) junctions, the tunneling conductance of a NIS junction in graphene is an oscillatory function of the effective barrier strength of the insulating region, in the limit of a thin barrier. The amplitude of these oscillations is maximum for aligned Fermi surfaces of the normal and superconducting regions and vanishes for a large Fermi surface mismatch. The zero-bias tunneling conductance, in sharp contrast to its counterpart in conventional NIS junctions, becomes maximum for a finite barrier strength. We also suggest experiments to test these predictions
Tunneling Conductance of Graphene NIS Junctions
No full textWe show that, in contrast with conventional normal metal-insulator-superconductor (NIS) junctions, the tunneling conductance of a NIS junction in graphene is an oscillatory function of the effective barrier strength of the insulating region, in the limit of a thin barrier. The amplitude of these oscillations is maximum for aligned Fermi surfaces of the normal and superconducting regions and vanishes for a large Fermi surface mismatch. The zero-bias tunneling conductance, in sharp contrast to its counterpart in conventional NIS junctions, becomes maximum for a finite barrier strength. We also suggest experiments to test these predictions
Elastic signatures of a spin-nematic
No full textWe study the elastic signatures -- renormalisation of sound velocity and
magnetostriction -- of the spin-nematic phase of a spin- magnet on a
triangular lattice described by the bilinear-biquadratic spin Hamiltonian. We
show that at low temperatures, the scattering of the acoustic phonons from the
Goldstone modes of the nematic phase lead to a powerlaw renormalisation of the
fractional change in the sound velocity, , as a function of temperature,
, i.e. as opposed to the same in the high
temperature paramagnet where . At the generically
discontinuous nematic transition, there is a jump in magnetostriction as well
as along with enhanced dependence on the magnetic field,
, near the nematic transition. These signatures can help positively
characterise the spin-nematic in general and in particular the realisation of
such a phase in the candidate material NiGaS
Temperature dependence of butterfly effect in a classical many-body System
No full textWe study the chaotic dynamics in a classical many-body system of interacting spins on the kagome lattice. We characterise many-body chaos via the butter y effect as captured by an appropriate out-of-time-ordered correlator. Due to the emergence of a spin liquid phase, the chaotic dynamics extends all the way to zero temperature. We thus determine the full temperature dependence of two complementary aspects of the butter y effect: the Lyapunov exponent and the butterfly speed, and study their interrelations with usual measures of spin dynamics such as the spin-diffusion constant and spin-autocorrelation time.Physic
