arXiv subjects

Partha Goswami

Publications and source records attributed to Partha Goswami.

53 records · Page 3Linked to original sources

arXiv2012

Change in Stacking Order and Lifshitz transition in Bi-layer Graphene

We consider the AB-(Bernal) stacking for the bi-layer graphene (BLG) system and assume that a perpendicular electric field is created by the external gates deposited on the BLG surface. In the basis (A1, B2, A2, B1) for the valley K and the basis (B2, A1, B1, A2) for the valley K', we show the occurrence of trigonal warping [ see A. S. Núñez et al.,arXiv:1012.4318], that is, splitting of the energy bands or the density of states on the kx - ky plane into four pockets comprising of the central part and three legs due to a (skew) interlayer hopping between A1 and B2. The hopping between A1 - B2 leads to a concurrent velocity v3in addition to the Fermi velocity vF. Our noteworthy outcome is that the above-mentioned topological change, referred to as the Lifshitz transition [I. M. Lishitz, Zh. Exp. Teor. Fiz., 38, 1565 (1960) (Sov. Phys. JETP 11, 1130 (1960));Y. Lemonik, I.L. Aleiner, C. Toke, and V.I. Fal'ko; arXiv:1006.1399], is entirely bias-tunable. Furthermore, the many-body effects, which is known to yield logarithmic renormalizations [C. Hwang, D. A. Siegel, Sung-Kwan Mo, W. Regan, A. Ismach, Y. Zhang, A. Zettl and A. Lanzara, arXiv:1208.0567] in the band dispersions of monolayer graphene, is found to have significant effect on the bias-tunability of this transition. We also consider the system where the A atoms of the two layers are over each other and the B atoms of the layers are displaced with respect to each other. The trigonal warping is found to be absent in this case. Instead, the Fermi energy density of states for zero bias corresponds to the inverted sombrero-like structure. The structure is found to get deformed due to the increase in the bias.

cond-mat.mes-hall↗

arXiv2012

Local Moment Formation by Vacancies in Mono-layer Graphene

We employ the Green's function technique to investigate the vacancy-induced quasi-localized magnetic moment formation in mono-layer graphene starting with the Dirac Hamiltonian, which focuses on the π- orbitals only, involving the nearest neighbor(NN)(t) and moderate second neighbor(SN)(t' < t/3) hopping integrals. The vacancy defect is modeled by the addition of the on-site perturbation potential to the Hamiltonian. We find that, when (t'/t) << 1, the vacancy induced π-state at the zero of energy(zero-mode state(ZMS)) does not inhabit the minority sub-lattice due to the strong scalar potential induced by the vacancy(the ZMSs get lodged in the majority sub-lattice) whereas, when (t'/t) is increased, the ZMS is somewhat suppressed. This shows that, not only the shift of the Fermi energy away from the linearly-dispersive Dirac points, the issue of this topological localization is also hinged on the ratio (t'/t). Furthermore, when a vacancy is present, the three sp2- hybridized σ states of each of the three nearest-neighbor carbon atoms, forming a carbon triangle surrounding the vacancy, are close to the Fermi energy (EF). The Hund's coupling between these σ electrons and the remaining electron which occupies the π state spin polarizes the π state leading to local moment formation close to EF. Since the system at the Fermi level has low electronic density, there is poor screening of such magnetic moments. This may lead to a high Curie temperature for such vacancy-induced moments.

cond-mat.mes-hall↗

arXiv2011

Time-reversal and parity violating superconducting state of Bi_2Sr_2CaCu_20_8

The time-reversal and parity violating superconducting(SC) phase of Bi2Sr2CaCu2O8 induced by a magnetic field (B), reported by Krishana et al[K. Krishana et al, Science 277, 83 (1997)] over the limited field range of 0.6T < B < 5T, is examined starting with the model free-energy functional predicted by Laughlin [R. B. Laughlin, Physica C 234, 280 (1994)]. On the basis of entropy considerations we show that the passage from the normal to this field-driven SC state at a non-zero temperatures (T) is possible if kBT \leq Δ0 where the zero-temperature gap \Delta0 ~ \hbarv/lB, v is the root-mean-square velocity of the d-wave node, and lB = \surd(\hbar/eB) is the magnetic length. The restriction may be construed as setting lower limit on the required field strength at a given temperature. We find that, over the field range mentioned, the specific heat coefficient exhibits \surd B- and the B-linear dependence (Volovick effect).

cond-mat.supr-con↗

arXiv2011

Discernable signature of QPT in a bilayer-quantum-well system at the filling fraction ν = 5/2 in the low temperature range (1K-100 K)

We consider the spin polarized fermions for the filling fraction 5/2 in a bi-layer quantum well system. Since the kinetic energy of the system in fractional quantum Hall states is totally quenched, the Hamiltonian describing the system comprises of the electron correlation and tunneling terms. The correlations are captured by the 'so-called' Haldane pseudo-potentials. We employ the finite-temperature formalism involving Matsubara propagators to deal with this Hamiltonian. We show that the system undergoes a zero-order quantum phase transition (QPT), at fixed charge imbalance regulatory parameter (CIRP) and constant layer separation as the inter-layer tunneling strength (ILTS) is increased, from the effective two-component state (two independent layers) to an effective single-component state (practically a single layer). At finite and constant ILTS, a transition from the latter state to the former state is also possible upon increasing the CIR parameter. We identify the order parameter to describe this QPT as a pseudo-spin component (analogous to the z-component of the single spin-1/2 operator S) and calculate the order parameter with the aid of the Matsubara propagators. The clear finger-print of this QPT is obtained up to temperature equal to 100 K.

cond-mat.mes-hall↗

arXiv2010

Fingerprint of QPT in a bilayer-quantum-well system at the filling fraction ν = 5/2 in the low temperature range (1K-100 K)

We consider the spin polarized fermions for the filling fraction 5/2 in a bi-layer quantum well system. Since the kinetic energy of the system in fractional quantum Hall states (FQHS) is totally quenched, the Hamiltonian describing the system comprises of the electron correlation and tunneling terms. The correlations are captured by the 'so-called' Haldane pseudo-potentials(HPP). We employ the finite-temperature formalism involving Matsubara propagators to deal with this Hamiltonian. We show that the system undergoes a zero-order quantum phase transition (QPT), at fixed charge imbalance regulatory(CIR) parameter and constant layer separation as the inter-layer tunneling (ILT)strength is increased, from the effective two-component state (two independent layers) to an effective single-component state (practically a single layer). At finite and constant ILT strength, a transition from the latter state to the former state is also possible upon increasing CIR parameter. We identify the order parameter to describe this QPT as a pseudo-spin component and calculate the order parameter with the aid of the Matsubara propagators. The clear finger-print of this QPT is obtained up to temperature equal to 100 K.

cond-mat.mes-hall↗

arXiv2010

Fermi pockets and quantum oscillations in specific heat of YBCO in the presence of disorder

We investigate a chiral d-density wave (CDDW) mean field model Hamiltonian in the momentum space suitable for the hole-doped cuprates, such as YBCO, in the pseudo-gap phase to obtain the Fermi surface(FS)topologies, including the anisotropy parameter(Épsilon) and the elastic scattering by disorder potential (|v0|). For Épsilon = 0, the chemical potential μ = - 0.27 eV for 10% doping level, and |v0| \geq |t| (where |t| = 0.25 eV is the first neighbor hopping), at zero/non-zero magnetic field (B), the FS on the first Brillouin zone are found to correspond to Fermi pockets around anti-nodal regions and barely visible patches around nodal regions. For Épsilon \neq 0, we find Pomeranchuk distortion of FS. We next relate our findings regarding FS to the magneto-quantum oscillations in the electronic specific heat. Since the nodal quasi-particle energy values for B = 0 are found to be greater than μ for |v0| \geq |t|, the origin of the oscillations for non-zero B corresponds to the Fermi pockets around anti-nodal regions. The oscillations are shown to take place for 17 T \leq B \leq 53 T and beyond in the weak disorder regime (|v0|=0.25eV) only.

cond-mat.supr-con↗

arXiv2010

Electronic viscosity in a multiple quantum well system

We calculate the electronic viscosity for a multiple quantum well structure in the presence of disorder potential(V = 4 meV), the electron-electron repulsion(U = 5-17 meV) and a strong magnetic field (B greater than or equal to 16.5 T) in the direction in which the electrons are trapped. This viscosity is different from the dissipation-less Hall viscosity which cannot take non-zero value in a time reversal invariant system. The Fermi energy density of states for the system has been calculated in the t-matrix approximation assuming low concentration of impurities. Our approach involves calculation of the density of viscosity, for temperature close to 0 K, on the Brillouin zone(BZ) followed by the numerical evaluation of the integral of viscosity density over the BZ. We show that (i) the viscosity coefficient is nearly proportional to B for given (V,U),and (ii) dissipation-less state, analogous to superfluidity, is possible for a critical value of U when (V, B) are given. We also calculate the entropy per particle (S) and show that the results comply with the KSS bound reported by Kovtun et al.

cond-mat.mes-hall↗

arXiv2009

Fermi surface topology and magneto-quantum oscillations for under-doped YBCO with hopping anisotropy

We investigate a chiral d-density wave (CDDW) mean field Hamiltonian in momentum space, which also includes the in-plane hopping anisotropy parameter e, for the under-doped YBCO to explore the possibility of quantum oscillations (QO) in the specific heat in the presence of a changing magnetic field (B). The inclusion is motivated by the experimental signature of nematic order found in cuprates in neutron scattering experiments of Hinkov et al. (see Ref.[10]). We obtain the Fermi surface topologies, without and with magnetic field background, including the elastic scattering by impurities in the investigation within t-matrix approximation framework. The topologies are found to be distinct from the Fermi arc picture: we find that the minimally gapped portion of Fermi surface forms closed loops (hole pockets) and not arcs. However, some k-points of the electron pockets are found to be associated with the negative density of state values (see Ref.[28]) when B and e are zero; for non-zero B and e slightly positive these pockets acquire positive DOS values. The latter thus seems to justify the inclusion of the hopping anisotropy in the Hamiltonian. We relate these findings to QO in specific heat. We show that the origin of the main frequency of the oscillations is the electron pocket of the Fermi surface at a magnetic field B equal to 50 T.

cond-mat.supr-con↗

arXiv2009

On Nernst effect and quantum oscillations in NCCO

We start with a mean field Hamiltonian in momentum space for NCCO to investigate the magnetic quantum oscillations (MQO) exhibited by this system in the normal state. We first obtain an expression for the entropy of the system which is useful to explain the Nernst effect. We next derive the density of states at Fermi energy and the longitudinal electrical conductivity expression to explore the possibility of MQO. We find that the slow (1/B)-oscillations, with frequency nearly 330 T, arise from the hole pockets at the optimal doping. The faster oscillations at slightly higher doping level is found to have the frequency nearly equal to 900 T.

cond-mat.supr-con↗

arXiv2009

Theoretical investigation of a CDDW Hamiltonian for YBCO to explore the possibility of quantum oscillations in specific heat

We investigate a chiral d-density wave (CDDW) mean field Hamiltonian in momentum space for the under-doped YBCO. We consider the commensurate density wave order which doubles the unit cell of the real space lattice and derive an expression for entropy in the CDDW state in the absence of magnetic field. We show that the transition to CDDW state gives rise to the occurrence of discontinuities in the entropy density difference between the CDDW state and the normal state at eight points(Hot spots) on the boundary of the reduced Brillouin zone. The chirality induced anomalous Nernst signal is shown to arise mainly from the points close to these Hot spots and not from the hole pockets as reported by the previous authors. We also show the possibility of quantum oscillations in the specific heat for the system under consideration in the presence of a changing magnetic field.

cond-mat.supr-con↗

arXiv2009

Investigation of chiral density wave mean field Hamiltonian

We start with the chiral density wave(CDW)mean field Hamiltonian in the momentum space for the pseudo-gapped state of YBCO in the absence of magnetic field, including the momentum conserving inter-layer tunneling matrix elements in the Hamiltonian for the tetragonal case, with the principal component of the order parameter corresponding to idx2-y2 and the secondary one to the dxy. With the eigenvalues and the eigenvectors of the Hamiltonian matrix we write down expression for Berry curvature(BC). Only two BC peaks of same magnitude and opposite signs are obtained at the nodal points of the reconstructed Fermi surface (RFS) for the pure idx2-y2-density wave case. We establish a relation between thermodynamic potential of the system and certain spectral functions for the CDW state. This yields an expression for entropy in closed form. We show that the transition to the CDW state is a first order one and the entropy per unit cell increases in this state. We also investigate anomalous Nernst effect theoretically.

cond-mat.supr-con↗

arXiv2009

Theoretical investigation of pseudo-gap state of YBCO

A momentum space, mean field d-density wave (DDW) Hamiltonian is investigated self-consistently. The pseudo-gapped(PG)state of YBCO is assumed to correspond to the pure DDW state. A relation between thermodynamic potential of the system and certain spectral weight functions is established. This yields an expression for entropy in DDW state in the absence of magnetic field. The relation is useful for deriving finite temperature thermodynamics of the system. We show that the PG transition is a first order one and the entropy per unit cell increases in this state. We also analyze the fermion occupancy, in the presence of magnetic field, at the anti-nodal points of the Fermi surface to estimate the frequency of quantum oscillations.

cond-mat.supr-con↗

arXiv2009

Investigation of confined electron gas in a quantum well structure with photovoltaic application

The growth-diection quantization of confined electron gas in a GaAs/AlGaAs based quantum well structure is obtained in the Kohn-Sham iterative computational scheme. The longitudinal conductance at low temperatures, in the presence of magnetic field applied in the same direction, is studied to confirm the presence of the bound-states obtained through this scheme. The sub-band absorption is calculated by a frequently used approximation modeling the absorption as a convolution between spectral functions. The absorption coefficient is used to obtain photovoltaic performance related parameters of a quantum well solar cell. When compared to a device without the quantum wells, it is found that the additional carrier generation in the wells due to extension of absorption edge into longer wavelengths brings about moderate enhancement in the values of these parameters.

cond-mat.mes-hall↗

arXiv2008

Optical absorption and emission of a quantum dot in the Kondo regime

A variant of the Anderson model, that describes hybridization between localized state (c-state) of a quantum dot and a Fermi sea conduction band, is investigated. We demonstrate that, as a function of the hybridization parameter v, the system undergoes a crossover from the state where the conduction band and the c-level are fully coupled to a state where these are decoupled. The c-electron spectrum, however, has a gap together with the presence of the Kondo peak in the former state. For the latter, we have a Mott-like localization where the c-electron spectrum again has a gap without the Kondo peak. Within this gap the conduction electrons fully recover the free band density of states and the effective hybridization is practically zero. Our main aim, however, is to study the emission and absorption in a quantum dot with strongly correlated Kondo ground state. We use the Green function equation of motion method for this purpose. We calculate the absorption/emission (A/E) spectrum in the Kondo regime through a symmetrized quantum autocorrelation function obtainable directly within perturbation theory using the Fermi golden rule approximation. The spectrum reveals a sharp, tall peak close to Kondo-Abrikosov-Suhl peak and a few smaller, distinguishable ones on either side. The former clearly indicates that the Kondo phenomenon has its impact on A/E (non-Kondo processes), which are driven by the coupling involving the dipole moment of quantum dot transitions reflecting the physical structure of the dot including the confinement potential, in the Kondo regime.

cond-mat.str-el↗

arXiv2008

Inducement of Spin-pairing and Correlated Semi-metallic State in Mott-Hubbard Quantum Dot Array

We model a quantum dot-array (with one electron per dot) comprising of two (or more than two) coupled dots by an extended Hubbard Hamiltonian to investigate the role played by the inter-dot tunneling amplitude td, together with intra-dot (U) and inter-dot(U1) coulomb repulsions, in the singlet / triplet bound state formation and evolution of the system from the Mott-insulator-like state to a correlated semi-metallic state via charge-bond-order route. In the presence of magnetic field, td is complex due to the appearance of Peierls phase factor. We introduce a short-ranged inter-dot capacitive coupling U0, assumed to be non-zero for nearest-neighbor dots only, for the bound state analysis. The study indicates that, while for the tunable parameter d = (2td/U0) greater than unity only the possibility of the triplet bound state formation exists, for d less than one both triplet and singlet states are possible. The bound states are formed due to tunneling and capacitive dot-bondings with coulomb interactions (U,U1) playing marginal role. The interaction U, however, is found to play, together with complex td, an important role in the evolution of the double quantum dot system from the insulator-like state to that of a correlated semi-metallic state through charge-bond-ordering route.

cond-mat.str-el↗

arXiv2008

Theoretical Investigation of Fermion Pairings in Three-band Extended Hubbard Model

The two-dimensional d-p model (or extended Hubbard model) on a square lattice is investigated for fermion pairing by a slave boson method. The inter-site d-fermion interaction is introduced additionally. The momentum space counterpart of this interaction is taken to be separable and expanded in terms of basis functions corresponding to mixed symmetry states. The investigation leads to anomalous pairing of fermion fields only for pure d-wave symmetry. The charge and spin ordering gaps appear in the single- particle spectrum when d(x,y) state is taken into account.The non-Fermi liquid behavior is found to be the prevalent one generally. The study yields a good qualitative account of the nodal-antinodal dichotomy. Two gaps appear in the normal state spectrum; the larger one corresponds to the sum and the smaller to the difference of spin and charge ordering gaps. This two-gap scenario is in qualitative agreement with recent experimental finding.

cond-mat.supr-con↗

arXiv2008

Thermal and Quantum Fluctuations Induced Additional Gap in Single-particle Spectrum of d-p model

The possibility of thermal and quantum fluctuations induced attractive interaction leading to a pairing gap Δ_tq in the single-particle spectrum of d-p model in the limit of a large N of fermion flavor is investigated analytically. This is an anomalous pairing gap in addition to the one with d-wave symmetry originating from partially screened, inter-site coulomb interaction. The motivation was to search for a hierarchy of multiple many-body interaction scales in high-Tc superconductor as suggested by recent experimental findings. The pairing gap anisotropy stems from more than one sources, namely, nearest neighbour hoppings and the p-d hybridization but not the coupling of the effective interaction. The temperature at which Δ_tq vanishes may be driven to zero by using a tuning parameter to have access to quantum criticality only when N >> 1. For the physical case N = 2, the usual coherent quasi-particle feature surfaces in the spectral weight everywhere in the momentum space below the pairing gap Δ_tq. Thus it appears that the reduction in spin degeneracy has the effect of masking quantum criticality.

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