arXiv subjects

V. N. Muthukumar

Publications and source records attributed to V. N. Muthukumar.

At least 19 recordsLinked to original sources

arXiv2007

Gutzwiller-RVB Theory of High Temperature Superconductivity: Results from Renormalised Mean Field Theory and Variational Monte Carlo Calculations

We review the Resonating Valence Bond (RVB) theory of high temperatur e superconductivity using Gutzwiller projected wave functions that incorporate strong correlations. After a general overview of the phenomenon of high temperature superconductivity, we discuss Anderson's RVB picture and its implementation by renormalised mean field theory (RMFT) and variational Monte Carlo (VMC) techniques. We review RMFT and VMC results with an emphasis on recent development s in extending VMC and RMFT techniques to excited states. We compare results obtained from these methods with angle resolved photoemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM). We conclude by summarising recent successes of this approach and discuss open problems that need to be solved for a consistent and complete description of high temperature superconductivity using Gutzwiller projected wave functions.

cond-mat.str-el↗

arXiv2006

Spontaneous breaking of the Fermi surface symmetry in the t-J model: a numerical study

We present a variational Monte Carlo (VMC) study of spontaneous Fermi surface symmetry breaking in the t-J model. We find that the variational energy of a Gutzwiller projected Fermi sea is lowered by allowing for a finite asymmetry between the x- and the y-directions. However, the best variational state remains a pure superconducting state with d-wave symmetry, as long as the underlying lattice is isotropic. Our VMC results are in good overall agreement with slave boson mean field theory (SBMFT) and renormalized mean field theory (RMFT), although apparent discrepancies do show up in the half-filled limit, revealing some limitations of mean field theories. VMC and complementary RMFT calculations also confirm the SBMFT predictions that many-body interactions can enhance any anisotropy in the underlying crystal lattice. Thus, our results may be of consequence for the description of strongly correlated superconductors with an anisotropic lattice structure.

cond-mat.str-el↗

arXiv2006

Determining the underlying Fermi surface of strongly correlated superconductors

The notion of a Fermi surface (FS) is one of the most ingenious concepts developed by solid state physicists during the past century. It plays a central role in our understanding of interacting electron systems. Extraordinary efforts have been undertaken, both by experiment and by theory, to reveal the FS of the high temperature superconductors (HTSC), the most prominent strongly correlated superconductors. Here, we discuss some of the prevalent methods used to determine the FS and show that they lead generally to erroneous results close to half filling and at low temperatures, due to the large superconducting gap (pseudogap) below (above) the superconducting transition temperature. Our findings provide a perspective on the interplay between strong correlations and superconductivity and highlight the importance of strong coupling theories for the characterization as well as the determination of the underlying FS in ARPES experiments.

cond-mat.supr-con↗

arXiv2006

Electronic structure of strongly correlated d-wave superconductors

We study the electronic structure of a strongly correlated d-wave superconducting state. Combining a renormalized mean field theory with direct calculation of matrix elements, we obtain explicit analytical results for the nodal Fermi velocity, v_F, the Fermi wave vector, k_F, and the momentum distribution, n_k, as a function of hole doping in a Gutzwiller projected d-wave superconductor. We calculate the energy dispersion, E_k, and spectral weight of the Gutzwiller-Bogoliubov quasiparticles, and find that the spectral weight associated with the quasiparticle excitation at the antinodal point shows a non monotonic behavior as a function of doping. Results are compared to angle resolved photoemission spectroscopy (ARPES) of the high temperature superconductors.

cond-mat.supr-con↗

arXiv2005

Particle number renormalization in almost half filled Mott Hubbard superconductors

The effects of the Gutzwiller projection on a BCS wave function with varying particle number are considered. We show that a fugacity factor has to be introduced in these wave functions when they are Gutzwiller projected, and derive an expression for this factor within the Gutzwiller approximation. We examine the effects of the projection operator on BCS wave functions by calculating the average number of particles before and after projection. We also calculate particle number fluctuations in a projected BCS state. Finally, we point out the differences between projecting BCS wave functions in the canonical and grand canonical schemes, and discuss the relevance of our results for variational Monte Carlo studies.

cond-mat.supr-con↗

arXiv2005

Bosonic resonating valence bond wave function for doped Mott insulators

We propose a new class of ground states for doped Mott insulators in the electron second-quantization representation. They are obtained from a bosonic resonating valence bond (RVB) theory of the t-J model. At half filling, the ground state describes spin correlations of the S=1/2 Heisenberg model very accurately. Its spin degrees of freedom are characterized by RVB pairing of spins, the size of which decreases continuously as holes are doped into the system. Charge degrees of freedom emerge upon doping and are described by twisted holes in the RVB background. We show that the twisted holes exhibit an off diagonal long range order (ODLRO) in the pseudogap ground state, which has a finite pairing amplitude, but is short of phase coherence. Unpaired spins in such a pseudogap ground state behave as free vortices, preventing superconducting phase coherence. The existence of nodal quasiparticles is also ensured by such a hidden ODLRO in the ground state, which is non-Fermi-liquid-like in the absence of superconducting phase coherence. Two distinct types of spin excitations can also be constructed. The superconducting instability of the pseudogap ground state is discussed and a d-wave superconducting ground state is obtained. This class of pseudogap and superconducting ground states unifies antiferromagnetism, pseudogap, superconductivity, and Mott physics into a new state of matter.

cond-mat.str-el↗

arXiv2005

On the evaluation of matrix elements in partially projected wave functions

We generalize the Gutzwiller approximation scheme to the calculation of nontrivial matrix elements between the ground state and excited states. In our scheme, the normalization of the Gutzwiller wave function relative to a partially projected wave function with a single non projected site (the reservoir site) plays a key role. For the Gutzwiller projected Fermi sea, we evaluate the relative normalization both analytically and by variational Monte-Carlo (VMC). We also report VMC results for projected superconducting states that show novel oscillations in the hole density near the reservoir site.

cond-mat.str-el↗

arXiv2005

Antiferromagnetism and d-wave superconductivity in (doped) Mott insulators: A wave function approach

We propose a class of wave functions that provide a unified description of antiferromagnetism and d-wave superconductivity in (doped) Mott insulators. The wave function has a Jastrow form and prohibits double occupancies. In the absence of holes, the wave function describes antiferromagnetism accurately. Off diagonal long range order develops at finite doping and the superconducting order parameter has d-wave symmetry. We also show how nodal quasiparticles and neutral spin excitations can be constructed from this wave function.

cond-mat.str-el↗

arXiv2003

Entanglement of a qubit with a single oscillator mode

We solve a model of a qubit strongly coupled to a massive environmental oscillator mode where the qubit backaction is treated exactly. Using a Ginzburg-Landau formalism, we derive an effective action for this well known localization transition. An entangled state emerges as an instanton in the collective qubit-environment degree of freedom and the resulting model is shown to be formally equivalent to a Fluctuating Gap Model (FGM) of a disordered Peierls chain. Below the transition, spectral weight is transferred to an exponentially small energy scale leaving the qubit coherent but damped. Unlike the spin-boson model, coherent and effectively localized behaviors may coexist.

cond-mat.mes-hall↗

arXiv2002

Quasiparticles as composite objects in the RVB superconductor

We study the nature of the superconducting state, the origin of d-wave pairing, and elementary excitations of a resonating valence bond (RVB) superconductor. We show that the phase string formulation of the t-J model leads to confinement of bare spinon and holon excitations in the superconducting state, though the vacuum is described by the RVB state. Nodal quasiparticles are obtained as composite excitations of spinon and holon excitations. The d-wave pairing symmetry is shown to arise from short range antiferromagnetic correlations.

cond-mat.str-el↗

arXiv2002

Spontaneous Vortex Phase in the Bosonic RVB Theory

In the description of spin-charge separation based on the phase string theory of the t-J model, spinon excitations are vortices in the superconducting state. Thermally excited spinons destroy phase coherence, leading to a new phase characterized by the presence of free spinon vortices at temperatures, T_c<T<T_v. The temperature scale T_v at which holon condensation occurs marks the onset of pairing amplitude, and is related to the spin pseudogap temperature T^*. The phase below T_v, called the spontaneous vortex phase, shows novel transport properties before phase coherence sets in at T_c. We discuss the Nernst effect as an intrinsic characterization of such a phase, in comparison with recent experimental measurements.

cond-mat.str-el↗

arXiv2001

Ginzburg-Landau theory of an RVB superconductor

We present a Ginzburg-Landau formulation of the bosonic resonating valence bond (RVB) theory of superconductivity. The superconducting order parameter is characterized by phase vortices that describe spinon excitations and the transition to the superconducting state occurs when such phase vortices (un)bind. We show that the boson RVB theory always leads to hc/2e flux quanta and that the presence of a trapped spin-1/2 moment inside a vortex core gives rise to observable consequences for the low temperature field dependent specific heat. We also show that the cores of magnetic fluxoids exhibit enhanced antiferromagnetic correlations.

cond-mat.str-el↗

arXiv2001

Spectral function of the electron in a superconducting RVB state

We present a model calculation of the spectral function of an electron in a superconducting resonating valence bond (RVB) state. The RVB state, described by the phase-string mean field theory is characterized by three important features: (i) spin-charge separation, (ii) short range antiferromagnetic correlations, and (iii) holon condensation. The results of our calculation are in good agreement with data obtained from Angle Resolved Photoemission Spectroscopy (ARPES) in superconducting Bi 2212 at optimal doping concentration.

cond-mat.supr-con↗

arXiv2001

Localization transition in the Mermin model

We study the dynamical properties of the Mermin model, a simple quantum dissipative model with a monochromatic environment, using analytical and numerical methods. Our numerical results show that the model exhibits a second order phase transition to a localized state before which the system is effectively decoupled from the environment. In contrast to the spin-boson model, the Mermin model exhibits an ``orthogonality catastrophe,'' defining the critical point, before dissipation has destroyed all coherent behavior. An analytic approach based on the Liouvillian technique, though successful in describing the phase diagram of spin-boson and related models, fails to capture this essential feature of the Mermin model.

cond-mat↗

arXiv2001

Nature of Spin Excitations in Two-dimensional Mott Insulators: Undoped Cuprates and Other Materials

We investigate the excitation spectrum of a two-dimensional resonating valence bond (RVB) state. Treating the $pi$-flux phase with antiferromagnetic correlations as a variational ground state, we recover the long wavelength magnon as an "RVB exciton". However, we find that this excitation does NOT exhaust the entire spectral weight and the high energy spectrum is dominated by fermionic excitations. The latter can be observed directly by inelastic neutron scattering and we predict their characteristic energy scales along different high symmetry directions in the magnetic Brillouin zone. We also interpret experimental results on two magnon Raman scattering and mid-infrared absorption within this scenario.

cond-mat.str-el↗

arXiv2000

Spin-charge separation in the single hole doped Mott antiferromagnet

The motion of a single hole in a Mott antiferromagnet is investigated based on the t-J model. An exact expression of the energy spectrum is obtained, in which the irreparable phase string effect [Phys. Rev. Lett. 77, 5102 (1996)] is explicitly present. By identifying the phase string effect with spin backflow, we point out that spin-charge separation must exist in such a system: the doped hole has to decay into a neutral spinon and a spinless holon, together with the phase string. We show that while the spinon remains coherent, the holon motion is deterred by the phase string, resulting in its localization in space. We calculate the electron spectral function which explains the line shape of the spectral function as well as the ``quasiparticle'' spectrum observed in angle-resolved photoemission experiments. Other analytic and numerical approaches are discussed based on the present framework.

cond-mat.str-el↗

arXiv1999

Charge Transport in Synthetic Metals

The phenomenology of charge transport in synthetic metals is reviewed. It is argued that the conventional quasiparticle picture and Boltzmann transport theory do not apply to these materials. The central ideas of Fermi liquid theory are reviewed, and the significant corrections produced by quasiparticle scattering from ferromagnetic spin fluctuations in liquid $^3$He are described. It is shown that Sr$_2$RuO$_4$ does not display the symptoms of a nearly-ferromagnetic Fermi liquid, so the source of its odd angular momentum pairing remains to be understood. The solution of an assisted-tunneling model of charge transport in quasi-one dimensional materials is described. This model has a quantum critical point and gives a resistivity that is linear in temperature or frequency, whichever is greater.

cond-mat.str-el↗

arXiv1998

Novel Nonreciprocal Acoustic Effects in Antiferromagnets

The possible occurrence of nonreciprocal acoustic effects in antiferromagnets in the absence of an external magnetic field is investigated using both (i) a microscopic formulation of the magnetoelastic interaction between spins and phonons and (ii) symmetry arguments. We predict for certain antiferromagnets the existence of two new nonreciprocal (non-time invariant) effects: A boundary-condition induced nonreciprocal effect and the occurrence of transversal phonon modes propagating in opposite directions having different velocities. Estimates are given and possible materials for these effects to be observed are suggested.

cond-mat↗