Tiruppattur Ramakrishnan | Indian Institute of Science (original) (raw)

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Papers by Tiruppattur Ramakrishnan

Physica C: Superconductivity, 2006

A model of high-temperature superconductivity in cuprates where delocalization of one-dimensional... more A model of high-temperature superconductivity in cuprates where delocalization of one-dimensional Wigner lattices of electron pairs is responsible for superconductivity is suggested. The milestones of the model are as follows:1.Symmetric polarization of oxygen ions in the CuO2 layer by two neighboring copper ions leading to formation of one-electron σ bonds between the ions.2.Asymmetric polarization of oxygen ions in the electric field around a CuO2 layer resulting in the excitation of singlet electron pairs of oxygen ions from the 2pz state into a (2pz3s) state, the singlet nature of the pair being preserved.3.Lowering of the CuO2 layer symmetry under doping and formation of quasi-one-dimensional chains of copper and oxygen ions bound by one-electron σ bonds.4.Formation of one-dimensional Wigner lattices of electron pairs due to a strong Coulomb interaction between these pairs.5.Formation of delocalized π orbitals along ion chains and delocalization of Wigner lattices of electron pairs at the temperature regarded as the temperature of transition to superconductivity.The model allows one to explain the co-existence of superconductivity and magnetism, orbital ion ordering, the role of local lattice distortions, the stripe structure of the electron density, and a quasi-one-dimensional character of superconductivity.

arXiv: Superconductivity, 2017

We study the Nernst effect in fluctuating superconductors by calculating the transport coefficien... more We study the Nernst effect in fluctuating superconductors by calculating the transport coefficient alphaxy\alpha_{xy}alphaxy in a phenomenological model where relative importance of phase and amplitude fluctuations of the order parameter is tuned continuously to smoothly evolve from an effective XY model to more conventional Ginzburg-Landau description. To connect with a concrete experimental realization we choose the model parameters appropriate for cuprate superconductors and calculate alphaxy\alpha_{xy}alphaxy and the magnetization bfM{\bf M}bfM over the entire range of experimentally accessible values of field, temperature and doping. We argue that alphaxy\alpha_{xy}alphaxy and bfM{\bf M}bfM are both determined by the equilibrium properties of the superconducting fluctuations (and not their dynamics) despite the former being a transport quantity. Thus, the experimentally observed correlation between the Nernst signal and the magnetization arises primarily from the correlation between alphaxy\alpha_{xy}alphaxy and bfM{\bf M}bfM. Further, ...

Bulletin of the American Physical Society, 2011

We describe here a minimal theory of tight binding electrons moving on the square planar Cu latti... more We describe here a minimal theory of tight binding electrons moving on the square planar Cu lattice of the hole-doped cuprates and mixed quantum mechanically with pairs of them (Cooper pairs). Superconductivity occurring at the transition temperature Tc is the long-range, d-wave symmetry phase coherence of these Cooper pairs. Fluctuations necessarily associated with incipient long-range superconducting order have a generic large-distance behaviour near Tc. We calculate the spectral density of electrons coupled to such Cooper pair fluctuations and show that features observed in Angle Resolved Photo Emission Spectroscopy (ARPES) experiments on different cuprates above Tc as a function of doping and temperature emerge naturally in this description. These include 'Fermi arcs' with temperature-dependent length and an antinodal pseudogap which fills up linearly as the temperature increases towards the pseudogap temperature. Our results agree quantitatively with experiment. Below Tc, the effects of nonzero superfluid density and thermal fluctuations are calculated and compared successfully with some recent ARPES experiments, especially the observed bending or deviation of the superconducting gap from the canonical d-wave form.

Annals of Physics, 2016

Using a recently proposed Ginzburg–Landau-like lattice free energy functional due to Banerjee et ... more Using a recently proposed Ginzburg–Landau-like lattice free energy functional due to Banerjee et al. (2011) we calculate the fluctuation diamagnetism of high-T c superconductors as a function of doping, magnetic field and temperature. We analyse the pairing fluctuations above the superconducting transition temperature in the cuprates, ranging from the strong phase fluctuation dominated underdoped limit to the more conventional amplitude fluctuation dominated overdoped regime. We show that a model where the pairing scale increases and the superfluid density decreases with underdoping produces features of the observed magnetization in the pseudogap region, in good qualitative and reasonable quantitative agreement with the experimental data. In particular, we explicitly show that even when the pseudogap has a pairing origin the magnetization actually tracks the superconducting dome instead of the pseudogap temperature, as seen in experiment. We discuss the doping dependence of the ‘ons...

We propose and develop the consequences of a theory in which the free energy $ F $ of a cuprate i... more We propose and develop the consequences of a theory in which the free energy $ F $ of a cuprate is expressed as a functional of the complex nearest neighbour spin singlet bond pair order parameter Deltaijexp(iphiij)\\ Delta_ {ij}\\ exp {(i\\ phi_ {ij})} Deltaijexp(iphiij). $ F $ is a sum of two terms $\\ sum_m (a\\ ...

Over the years, Angle Resolved Photo Emission Spectroscopy (ARPES) has uncovered a number of unus... more Over the years, Angle Resolved Photo Emission Spectroscopy (ARPES) has uncovered a number of unusual spectral properties of electrons with definite in-plane momenta near the Fermi energy in the hole doped cuprates. We describe here a minimal theory of tight binding electrons moving on the square planar Cu lattice of the cuprates, mixed quantum mechanically with pairs of them (Cooper

Materials Science Forum, 1985