arXiv subjects

Patrick A. Lee

Publications and source records attributed to Patrick A. Lee.

At least 163 records · Page 9Linked to original sources

arXiv2003

Thermodynamic and Transport Measurements on Superconducting Na_xCoO_2 + yH_2O Single Crystals Prepared by Electrochemical De-intercalation

Superconducting single crystal samples of Na_xCoO_2 + yH_2O have been produced using an electrochemical technique which dispenses with the usual bromine chemical de-intercalation step and permits much more precise control of the Na content. After careful hydration, nearly single-phase crystals have been obtained in which over 90% of the sample's volume corresponds to the superconducting Na_{0.3}CoO_2 + 1.3H_2O structure. Susceptibility and specific heat measurements confirm that bulk superconductivity has been achieved. The extracted normal state density of states indicates Fermi-liquid behavior with strong mass enhancement and a modest Wilson ratio. Measurements of H_{c2} for H||c and H||ab reveal significant anisotropy. The estimated value of H_{c2} for H||c yields a coherence length of ~100 A, consistent with an extremely narrow bandwidth.

cond-mat.str-el↗

arXiv2003

Possible effects of charge frustration in Na$_x$CoO$_2$: bandwidth suppression, charge orders and resurrected RVB superconductivity

Charge frustration due to further neighbor Coulomb repulsion can have dramatic effects on the electronic properties of Na$_x$CoO$_2$ in the full doping range. It can significantly reduce the effective mobility of the charge carriers, leading to a low degeneracy temperature $ε_F \lesssim T$. Such strongly renormalized Fermi liquid has rather unusual properties--from the point of view of the ordinary metals with $ε_F \gg T$--but similar to the properties that are actually observed in the Na$_x$CoO$_2$ system. For example, we show that the anomalous thermopower and Hall effect observed in Na$_{0.7}$CoO$_2$ may be interpreted along these lines. If the repulsion is strong, it can also lead to charge order; nevertheless, away from the commensurate dopings, the configurational constraints allow some mobility for the charge carriers, i.e., there remains some ``metallic'' component. Finally, the particularly strong bandwidth suppression around the commensurate $x=1/3$ can help resurrect the RVB superconductivity, which would otherwise not be expected near this high doping. These suggestions are demonstrated specifically for a $tJ$-like model with an additional nearest neighbor repulsion.

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arXiv2003

Staggered flux vortices and the superconducting transition in the layered cuprates

We propose an effective model for the superconducting transition in the high-T_c cuprates motivated by the SU(2) gauge theory approach. In addition to variations of the superconducting phase we allow for local admixture of staggered flux order. This leads to an unbinding transition of vortices with staggered flux core that are energetically preferable to conventional vortices. Based on parameter estimates for the two-dimensional t-J model we argue that the staggered flux vortices provide a way to understand a phase with a moderate density of mobile vortices over a large temperature range above T_c that yet exhibits otherwise normal transport properties. This picture is consistent with the large Nernst signal observed in this region.

cond-mat↗

arXiv2003

Pseudogap and Competing States in Underdoped Cuprates

I shall argue that the high Tc problem is the problem of doping into a Mott insulator. Furthermore, the well documented pseudo-gap phenomenon in underdoped cuprates holds the key to understanding this physics. Phase fluctuation alone cannot explain this phenomenon, but there is a clear need to identify a competing state which lives in the vortex core. The staggered flux state is a good candidate for the competing state and experimental tests of these ideas will be discussed.

cond-mat↗

arXiv2003

Weak-Field Thermal Hall Conductivity in the Mixed State of d-Wave Superconductors

Thermal transport in the mixed state of a d-wave superconductor is considered within the weak-field regime. We express the thermal conductivity, $κ_{xx}$, and the thermal Hall conductivity, $κ_{xy}$, in terms of the cross section for quasiparticle scattering from a single vortex. Solving for the cross section (neglecting the Berry phase contribution and the anisotropy of the gap nodes), we obtain $κ_{xx}(H,T)$ and $κ_{xy}(H,T)$ in surprisingly good agreement with the qualitative features of the experimental results for YBa$_{2}$Cu$_{3}$O$_{6.99}$. In particular, we show that the simple, yet previously unexpected, weak-field behavior, $κ_{xy}(H,T) \sim T\sqrt{H}$, is that of thermally-excited nodal quasiparticles, scattering primarily from impurities, with a small skew component provided by vortex scattering.

cond-mat.supr-con↗

arXiv2003

Field-induced Antiferromagnetism and the Kondo Insulator-to-Metal Transition

The Kondo lattice model, augmented by a Zeeman term, serves as a useful model of a Kondo insulator in an applied magnetic field. A variational mean field analysis of this system on a square lattice, backed up by quantum Monte Carlo calculations, reveals an interesting separation of magnetic field scales. For Zeeman energy comparable to the Kondo energy, the spin gap closes and the system develops transverse staggered magnetic order. The charge gap, however, survives up to a higher hybridization energy scale, at which point the canted antiferromagnetism is suppressed and the system becomes metallic. Quantum Monte Carlo simulations support this mean field scenario. An interesting rearrangement of spectral weight with magnetic field is found.

cond-mat.str-el↗

arXiv2003

Doped dimerized spin chains: Another path to superconductivity

This paper has been withdrawn by the authors, since we have come to believe that the present approach is not convergent at small doping. We will shortly submit an improved theory of the tJJ' model at small J/t in the dilute hole limit. This will be a new submission will go by a different submission number.

cond-mat.str-el↗

arXiv2003

New Collective Modes in the Superconducting Ground States in the Gauge Theory Description of the Cuprates

In the slave boson mean field treatment of the t-J model, the ground state for small doping is a d-wave superconductor. A conventional superconductor has collective modes associated with the amplitude and phase of the pairing order parameter. Here, the hopping matrix element $χ_{ij}$ is also a mean field order parameter. We therefore expect that new collective modes will be introduced with these additional complex degrees of freedom. We compute the new collective modes and their spectral functions by numerically diagonalizing the matrix which describes the fluctuations about the mean field solution. We also show that the SU(2) gauge theory formulation allows us to predict and classify these collective modes. Indeed, the SU(2) formulation is essential in order to avoid spurious collective modes as the doping $x$ goes to zero. The most important new collective modes are the $θ$ mode and the longitudinal and transverse $ϕ$ gauge modes. The $θ$ mode corresponds to fluctuations of the staggered flux phase and creates orbital current fluctuations. The $ϕ$ gauge modes correspond to out-of-phase fluctuations of the {\em amplitudes} of the pairing and the hopping matrix elements. We compute the neutron scattering cross-section which couples to the $θ$ mode and inelastic X-ray scattering cross-section which couples to the fluctuation in the real part of $χ_{ij}$. In addition, we show that the latter fluctuation at $(π,π)$ may be coupled to the buckling phonon mode in the LTO phase of LSCO and may be detected optically. Experimental searches of these collective modes will serve as important tests of this line of attack on the high T$_c$ problem.

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arXiv2003

Staggered-flux normal state in the weakly doped t-J model

A normal (non-superconducting) ground state of the t-J model may be variationally approximated by a Gutzwiller-projected wave function. Within this approximation, at small hole doping near half-filling, the normal state favors staggered-flux ordering. Such a staggered-flux state may occur in vortex cores of underdoped high-temperature cuprate superconductors. From comparing the energies of the staggered-flux state and of the superconducting state, we numerically obtain the condensation energy. Extracting the superfluid density directly from the projected superconducting wave function, we can also estimate the coherence length at zero temperature.

cond-mat.supr-con↗

arXiv2002

S=1/2 chains and spin-Peierls transition in TiOCl

We study TiOCl as an example of an S=1/2 layered Mott insulator. From our analysis of new susceptibility data, combined with LDA and LDA+U band structure calculations, we conclude that orbital ordering produces quasi-one-dimensional spin chains and that TiOCl is a new example of Heisenberg-chains which undergo a spin-Peierls transition. The energy scale is an order of magnitude larger than that of previously known examples. The effects of non-magnetic Sc impurities are explained using a model of broken finite chains.

cond-mat.str-el↗

arXiv2002

Staggered flux fluctuations and the quasiparticle scattering rate in the SU(2) gauge theory of the t-J model

We study staggered flux fluctuations around the superconducting state of the SU(2) meanfield theory for the two-dimensional t-J-model and their effect on the electron spectral function. The quasiparticle peaks near the saddlepoints get strongly broadened and partially wiped out by these fluctuations while the quasiparticle peaks near the nodes of the d-wave gap are preserved over a wide parameter range. The strength of these effects is governed by an energy scale that decreases towards zero doping and that is related to the energy splitting between the SU(2)-related superconducting and staggered flux meanfield states.

cond-mat.str-el↗

arXiv2002

Orbital currents and cheap vortices in underdoped cuprates

In the past several years, we have developed a theory for the underdoped cuprates in collaboration with X.-G. Wen, based on an SU(2) formulation of the t-J model. In this formulation, the staggered flux state plays a central role as the progenitor of the Neel state at half-filling and a close competitor to the d-wave superconductor with small doping where it is characterized by staggered orbital currents. We found support for this point of view when we discovered fluctuating orbital currents in the Gutzwiller projected BCS wavefunction. We shall argue that low-energy vortices, where the staggered flux state is stabilized in the core, are needed to explain many of the unusual properties of underdoped cuprates. Proposed experiments to look for these orbital currents will be discussed.

cond-mat↗

arXiv2002

Why are Orbital Currents Central to High Tc Theory?

We explain qualitatively why the staggered flux state plays a central role in the SU(2) formulation of the t-J model, which we use to model the pseudogap state in underdoped cuprates. This point of view is supported by studies of projected wavefunctions. In addition to staggered orbital current correlations, we present here for the first time results of correlations involving hole and spin chirality and show that the two are closely related. The staggered flux state allows us to construct cheap and fast vortices, which may hold the key to explaining the many anomalous properties of the normal state.

cond-mat.supr-con↗

arXiv2002

Local Moment Formation in the Superconducting State of a Doped Mott Insulator

A microscopic theory is presented for the local moment formation near a non-magnetic impurity or a copper defect in high-T_c superconductors. We use a renormalized meanfield theory of the t-J model for a doped Mott insulator and study the fully self-consistent, spatially unrestricted solutions of the d-wave superconducting (SC) state in both the spin S=0 and S=1/2 sectors. We find a transition from the singlet d-wave SC state to a spin doublet SC state when the renormalized exchange coupling exceeds a doping dependent critical value. The induced S=1/2 moment is staggered and localized around the impurity. It arises from the binding of an S=1/2 nodal quasiparticle excitation to the impurity. The local density of states spectrum is calculated and connections to NMR and STM experiments are discussed.

cond-mat.str-el↗

arXiv2002

Anomalous Spin Excitation Spectrum of the Heisenberg Model in a Magnetic Field

Making the assumption that high energy fermions exist in the two dimensional spin-1/2 Heisenberg antiferromagnet we present predictions based on the pi-flux ansatz for the dynamic structure factor when the antiferromagnet is subject to a uniform magnetic field. The main result is the presence of gapped excitations in a momentum region near (pi,pi) with energy lower than that at (pi,pi). This is qualitatively different from spin wave theory predictions and may be tested by experiments or by quantum Monte Carlo.

cond-mat.str-el↗

arXiv2002

Orbital Currents in Underdoped Cuprates

Orbital currents, either fluctuating or static, have emerged as promising candidates for a description of the pseudogap state in underdoped cuprates. I shall review the evolution of these ideas and describe some experiments which have been proposed to test the existence of orbital currents.

cond-mat.supr-con↗

arXiv2001

Energy Scales in the Local Magnetic Excitation Spectrum of YBCO

The wave-vector integrated dynamical spin susceptibility chi_{2D}(omega) of YBCO cuprates is considered. chi_{2D} is calculated in the superconducting state from a renormalized mean-field theory of the t-t'-J-model, based on the slave-boson formulation. Besides the well-known 41 meV resonance a second, much broader peak (`hump') appears in Im chi_{2D}. It is caused by particle-hole excitations across the maximum gap Delta^0. In contrast to the resonance, which moves to lower energies when the hole filling is reduced from optimal doping, the position of this `hump' at approx. 2 Delta^0 stays almost unchanged. The results are in reasonable agreement with inelastic neutron-scattering experiments.

cond-mat.str-el↗

arXiv2001

Microwave Conductivity due to Scattering from Extended Linear Defects in d-Wave Superconductors

Recent microwave conductivity measurements of detwinned, high-purity, slightly overdoped YBa$_{2}$Cu$_{3}$O$_{6.993}$ crystals reveal a linear temperature dependence and a near-Drude lineshape for temperatures between 1 and 20 K and frequencies ranging from 1 to 75 GHz. Prior theoretical work has shown that simple models of scattering by point defects (impurities) in d-wave superconductors are inconsistent with these results. It has therefore been suggested that scattering by extended defects such as twin boundary remnants, left over from the detwinning process, may also be important. We calculate the self-energy and microwave conductivity in the self-consistent Born approximation (including vertex corrections) for a d-wave superconductor in the presence of scattering from extended linear defects. We find that in the experimentally relevant limit ($Ω, 1/τ\ll T \ll Δ_{0}$), the resulting microwave conductivity has a linear temperature dependence and a near-Drude frequency dependence that agrees well with experiment.

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