Searcharxiv⌕ Search

arXiv subjects

G. Baskaran

Publications and source records attributed to G. Baskaran.

At least 73 records · Page 4Linked to original sources
arXiv2003

An Electronic Model for $CoO_2$ layer based systems: Chiral RVB metal and Superconductivity

Takada et al. have reported superconductivity in layered $Na__x CoO_2.yH_2O$ ($T_c \approx5 K$) and more recently Wen et al. in $A_xCoO_{2+δ}$ ($A = Na,K$)(\tc$\approx~31 K$). We model a reference neutral \cob layer as an orbitally non-degenerate spin-\half antiferromagnetic Mott insulator on a triangular lattice and $Na__x CoO_2.yH_2O$ and $A_xCoO_{2+δ}$ as electron doped Mott insulators described by a t-J model. It is suggested that at optimal doping chiral spin fluctuations enhanced by the dopant dynamics leads to a d-wave superconducting state. A chiral RVB metal, a PT violating state with condensed RVB gauge fields, with a possible weak ferromagnetism and low temperature p-wave superconductivity are also suggested at higher dopings.

cond-mat.str-el↗
arXiv2002

Mott Insulator to high \tc Superconductor via Pressure: Resonating Valence Bond theory and prediction of new Systems

Mott insulator superconductor transition, via pressure and no external doping, is studied in orbitally non degenerate spin-\half systems. It is presented as another RVB route to high \tc superconductivity. We propose a `strong coupling' hypothesis which helps to view first order Mott transition as a self doping process that also preserves superexchange on metal side . We present a generalized t-J model where a conserved $N_0$ doubly occupied ($e^-$) sites and $N_0$ empty sites ($e^+$) hop in the background of $N - 2 N_0$ singly singly occupied (neutral) sites in a lattice of N sites. An equivalence to the regular t-J model is made and some old and new systems are predicted to be candidates for pressure induced high \tc superconductivity.

cond-mat.str-el↗
arXiv2001

Predicting a Gapless Spin-1 Neutral Collective Mode branch for Graphite

Using the standard tight binding model of 2d graphite with short range electron repulsion, we find a gapless spin-1, neutral collective mode branch {\em below the particle-hole continuum} with energy vanishing linearly with momenta at the $Γ$ and $K$ points in the BZ. This spin-1 mode has a wide energy dispersion, 0 to $\sim 2 eV$ and is not Landau damped. The `Dirac cone spectrum' of electrons at the chemical potential of graphite generates our collective mode; so we call this `spin-1 zero sound' of the `Dirac sea'. Epithermal neutron scattering experiments, where graphite single crystals are often used as analyzers (an opportunity for `self-analysis'!), and spin polarized electron energy loss spectroscopy (SPEELS) can be used to confirm and study our collective mode.

cond-mat.str-el↗
arXiv2001

Modelling Two-Roton Bound State Formation in Fractional Quantum Hall System

Composite Fermion approach using extensive and parallalized numerical analysis has recently established a two-roton bound state as the lowest energy long wavelength neutral excitation of FQHE for finite particle (N~30) system. By focussing on the "oriented dipole" character of magneto roton, we model the two roton problem and solve it variationally (analytically) to find a two-roton bound state with binding energy which is in good agreement with the composite fermion numerical results.

cond-mat.mes-hall↗
arXiv2001

Theory of High \tc Ferromagnetism in $SrB_6$ family: A case of Doped Spin-1 Mott insulator in a Valence Bond Solid Phase

Doped divalent hexaborides such as $Sr_{1-x}La_xB_6$ exhibit high \tc ferromagnetism. We isolate a degenerate pair of $2p$-orbitals of boron with two valence electrons, invoke electron correlation and Hund coupling, to suggest that the undoped state is better viewed as a spin-1 Mott insulator; it is predicted to be a type of 3d Haldane gap phase with a spin gap $\sim 0.1 eV$, much smaller than the charge gap of $ > 1.0 eV$ seen in ARPES. The experimentally seen high \tc `ferromagnetism' is argued to be a complex magnetic order in disguise - either a canted 6-sublattice AFM ($\approx 120^0$) order or its quantum melted version, a chiral spin liquid state, arising from a type of double exchange mechanism.

cond-mat.str-el↗
arXiv2001

RVB Contribution to Superconductivity in $MgB_2$

We view $MgB_2$ as electronically equivalent to (non-staggered) graphite ($B^-$ layer) that has undergone a zero gap semiconductor to a superconductor phase transition by a large c-axis (chemical) pressure due to $Mg^{++}$ layers. Further, like the \ppi bonded planar organic molecules, graphite is an old resonating valence bond (RVB) system. The RVB's are the `preexisting cooper pairs' in the `parental' zero gap semiconducting $B^-$ (graphite) sheets that manifests themselves as a superconducting ground state of the transformed metal. Some consequences are pointed out.

cond-mat.str-el↗
arXiv2000

Spinon deconfinement above a finite energy gap in 2d Quantum Heisenberg Antiferromagnets

The familiar spin-1/2 quantum Heisenberg antiferromanget in a 2d square lattice is shown, within the non linear sigma model approximations, to be another novel state of matter that has excitations with fractional quantum numbers above a finite energy gap. 1-skyrmion with an energy $\approx 2πJ$ is shown to be made of two `deconfined spinons' or `SU(2) vortices'. The many skyrmion operator and the wave functions that we have found are strikingly similar to quantum Hall quasi particle operators and wave functions. We also predict the presence of finite energy `spin-S spinon' for a general spin-S Heisenberg antiferromagnets in 2d. Some consequences are briefly discussed.

cond-mat.str-el↗
arXiv2000

Quantum Number Fractionization in Cuprates: U(1) RVB Gauge Theory and Senthil Fisher Constructions

We critically look at Senthil Fisher constructions of quantum number fractionization in cuprates. The first construction, while mathematically correct is of limited relevance to cuprates. The second one has a missing local U(1) symmetry. Once this aspect is repaired, it reveals itself as the U(1) RVB gauge theory construction of quantum number fractionization. An approximate local $Z_2$ symmetry arises in the spin sector for a reason very different from Senthil and Fisher's. The charge sector continues to have the local U(1) symmetry and the metallic spin gap phase of the 2d cuprates may, in principle, carry (irrational) fractional charge $e^* \neq e$ obeying Haldane's fractional exclusion statistics like the 1d models.

cond-mat.str-el↗
arXiv2000

The Principle of Valence Bond Amplitude Maximization in Cuprates: How it breeds Superconductivity, Spin and Charge Orders

A simple microscopic principle of `Valence bond (nearest neighbor singlet) amplitude maximization '(VBAM) is shown to be present in undoped and optimally doped cuprates and unify the very different orderings such as antiferromagnetism in the Mott insulator and the robust superconductivity accompanied by an enhanced charge and stripe correlations in the optimally doped cuprates. VBAM is nearly synonymous with the energy minimization principle. It is implicit in the RVB theory and thereby makes the predictions of RVB mean field theory of superconductivity qualitatively correct.

cond-mat.str-el↗
arXiv2000

Condensed Matter Physics - Biology Resonance

The field of condensed matter physics had its genesis this century and it has had a remarkable evolution. A closer look at its growth reveals a hidden aim in the collective consciousness of the field - a part of the development this century is a kind of warm up exercise to understand the nature of living condensed matter, namely the field of biology, by a growing new breed of scientists in the coming century. Through some examples the vitality of this interaction will be pointed out.

cond-mat↗
arXiv1999

Competition between Superconductivity and Charge Stripe Order in High Tc Cuprates

Members of the high Tc cuprate family (with defect free $CuO_2$ planes) are suggested to have an `{\em Intrinsic single layer superconducting} Tc', larger than the experimentally observed Tc, at a given hole concentration. This difference occurs to varying degrees among the different members, due to an anomalous response of the d-wave superconducting state to self generated charge perturbations. In particular, any quasi elastic charge stripe order, arising from electron-electron and electron- lattice coupling, can suppress the large intrinsic superconducting Tc. Existing experimental results on the wide Tc variation in the single layer cuprate families, such as the low Tc ($\approx 38 K, 33 K$) of LSCO and Bi-2201 compared to high Tc ($\approx 95 K, 98 K$) of Tl-2201 and Hg-1201, the strain induced increase in Tc (25 to 50 K) observed in LSCO are qualitatively explained by our mechanism. We predict that under proper epitaxial strain the Tc of LSCO and Bi-2201 should increase from 30's all the way upto the intrinsic Tc $\sim $ 90 K, at optimal doping.

cond-mat.str-el↗
arXiv1999

Where are we in the theory of High Temperature Superconductors ?

In this talk I will briefly review our present theoretical understanding of some of the important issues in the high Tc cuprates. In view of its success, at a qualitative level and some times quantitative level, the theory initiated by Anderson and developed further by him and collaborators will be discussed. Many issues that still challenges us will be pointed out.

cond-mat↗
arXiv1999

K S Krishnan and the early experimental evidences for the Jahn-Teller Theorem

Jahn-Teller theorem, proposed in 1937, predicts a distortional instability for a molecule that has symmetry based degenerate electronic states. In 1939 Krishnan emphasized the importance of this theorem for the arrangement of water molecules around the transition metal or rare earth ions in aqueous solutions and hydrated saltes, in a short and interesting paper published in Nature by pointing out atleast four existing experimental results in support of the theorem. This paper of Krishnan has remained essentially unknown to the practitioners of Jahn-Teller effect, eventhough it pointed to the best experimental results that were available, in the 30's and 40's, in support of Jahn-Teller theorem. Some of the modern day experiments are also in conformity with some specific suggestions of Krishnan.

cond-mat↗
arXiv1995

Resonant Pair Tunneling and Singular effects of c-axis disorder in cuprates and organic superconductors

In the interlayer pair tunneling (ILPT) theory of superconductivity the large scale $T_c$ has its origin in the k-space locality of the inter layer pair tunneling matrix elements. We reinterpret the same physics as a process of resonant pair tunneling and illustrate it through cooper pair analysis. This interpretation is used to give a mechanism which leads to a singular suppression of $T_c$ as function of c-axis(off plane/axis) disorder. In this mechanism the non resonant tunneling processes arising from the c-axis disorder in general contributes a pair binding energy which is reduced by a factor $\frac{T_J}{ε_F}$, where $T_J$ is the interlayer pair tunneling matrix element and $ε_F$ is the fermi energy. This leads to a simple theorem which states that the scale of $T_c$ is controlled by the space average value of the bare one electron interlayer hoping matrix element. After briefly discussing that the ET and TMTSF molecule based organic superconductors are strongly correlated narrow band systems, the dramatic reduction of $T_c$ by anion disorder in organic superconductors is explained by our mechanism. Off plane disorder effects in some of the cuprates are also discussed

cond-mat↗
arXiv1995

Symmetry of the Gap Function in High-$T_c$ Superconductors

One of the most debated issues related to high-$T_c$ superconductivity is the symmetry of the Cooper pair or the gap function. In this report, we present numerical results regarding the gap function in strongly correlated electron systems using $t-J$ and Hubbard models in one and two dimensions. To this end, we use exact diagonalization to study the ground states of 8- and 16-site clusters consisting of single or coupled layers. We calculate a reduced two-particle density matrix in momentum space which is a measure of the gap function. We then analyze the eigenvectors of this density matrix, which display the possible Cooper pair symmetries. The eigenvector corresponding to the largest eigenvalue indicates a vanishing gap on the Fermi surface (which is in favour of odd-gap pairing) although $d_{x^2-y^2}$ symmetry is seen to be a very close contestant in many of the cases.

cond-mat↗
arXiv1995

Superconducting Gap Nodal Surface and Fermi Surface: their partial overlap in cuprates

Electron correlation in cuprates leads to a global constraint $\sum_{\bf k} Δ_{\bf k} = 0$ on the gap function $Δ_{\bf k}$ resulting in a gap nodal surface. We give physical arguments supported by numerical results and discuss some experimental results to argue that correlations also lead to a local constraint on charge fluctuations in ${\bf k}$-space close to the Fermi surface, which may result in a substantial overlap of the Fermi surface with the gap nodal surface.

cond-mat↗