arXiv subjects

O. Vafek

Publications and source records attributed to O. Vafek.

11 recordsLinked to original sources

arXiv2015

Thermodynamic signature of a magnetic-field-driven phase transition within the superconducting state of an underdoped high-temperature superconductor

More than a quarter century after the discovery of the high temperature superconductor (HTS) YBa$_2$Cu$_3$O$_{6+δ}$ (YBCO) studies continue to uncover complexity in its phase diagram. In addition to HTS and the pseudogap there is growing evidence for multiple phases with boundaries which are functions of temperature ($T$), doping (p) and magnetic field. Here we report the low temperature electronic specific heat (C$_{elec}$) of YBCO6.47 (p=0.08) up to a magnetic field (H) of 34.5 teslas (T), a poorly understood region of the underdoped H-$T$-p phase space. We observe two regimes: below a characteristic magnetic field H'$\approx$10 T, C$_{elec}/T$ obeys an expected H$^{1/2}$ behavior, however, near H' there is a sharp inflection followed by a linear-in-H behavior. H' rests deep within the superconducting phase and the linear-in-H behavior is observed in the zero resistance regime. In the limit of zero temperature, C$_{elec}/T$ is proportional to the zero-energy electronic density of states. Thus this inflection is evidence of a magnetic-field-driven quantum phase transition.

cond-mat.supr-con↗

arXiv2013

Cyclotron resonance of single valley Dirac fermions in gapless HgTe quantum well

We report on Landau level spectroscopy studies of two HgTe quantum wells (QWs) near or at the critical well thickness, where the band gap vanishes. In magnetic fields up to $B$=16T, oriented perpendicular to the QW plane, we observe a $\sqrt{B}$ dependence for the energy of the dominant cyclotron resonance (CR) transition characteristic of two-dimensional Dirac fermions. The dominant CR line exhibits either a single or double absorption lineshape for the gapless or gapped QW. Using an effective Dirac model, we deduce the band velocity of single valley Dirac fermions in gapless HgTe quantum wells, $v_F=6.4 \times10^5$ m/s, and interpret the double absorption of the gapped QW as resulting from the addition of a small relativistic mass.

cond-mat.mes-hall↗

arXiv2010

Planes, Chains, and Orbits: Quantum Oscillations and High Magnetic Field Heat Capacity in Underdoped YBCO

The underlying physics of the magnetic-field-induced resistive state in high temperature cuprate superconductors remains a mystery. One interpretation is that the application of magnetic field destroys the d-wave superconducting gap to uncover a Fermi surface that behaves like a conventional (i.e.Fermi Liquid) metal (1). Another view is that an applied magnetic field destroys long range superconducting phase coherence, but the superconducting gap amplitude survives (2, 3). By measuring the specific heat of ultra-clean YBa2Cu3O6.56 (YBCO 6.56), we obtain a measure of the quasi-particle density of states from the superconducting state well into the magnetic-field-induced resistive state. We have found that at very high magnetic fields the specific heat exhibits both the conventional temperature dependence and quantum oscillations expected for a Fermi Liquid. On the other hand, the magnetic field dependence of the quasi-particle density of states follows a \sqrt{H} behavior that persists right through the zero-resistance transition, evidencing the fully developed d-wave superconducting gap over the entire magnetic field range measured. The coexistence of these two phenomena pose a rigorous thermodynamic constraint on theories of high-magnetic-field resistive state in the cuprates.

cond-mat.supr-con↗

arXiv2007

Quantum Oscillations in the mixed state of d-wave superconductor

We show that the low-energy density of quasiparticle states in the mixed state of ultra-clean d-wave superconductors is characterized by pronounced quantum oscillations in the regime where the cyclotron frequency $\hbarω_c \ll Δ_0$, the d-wave pairing gap. Such oscillations as a function of magnetic field B are argued to be due to the internodal scattering of the d-wave quasiparticles near wavevectors $(\pm k_D,\pm k_D)$ by the vortex lattice as well as their Zeeman coupling. The periodicity of the oscillations is set by the condition $k_D \sqrt{hc/(eB)} \equiv k_D' \sqrt{hc/(eB')}\pmod {2π}$. We find that there is additional structure within each period which grows in complexity as the Dirac node anisotropy increases.

cond-mat.supr-con↗

arXiv2002

QED3 theory of pairing pseudogap in cuprates: From d-wave superconductor to antiferromagnet via "algebraic" Fermi liquid

High-$T_c$ cuprates differ from conventional superconductors in three crucial aspects: the superconducting state descends from a strongly correlated Mott-Hubbard insulator, the order parameter exhibits d-wave symmetry and superconducting fluctuations play an all important role. We formulate a theory of the pseudogap state in the cuprates by taking the advantage of these unusual features. The effective low energy theory within the pseudogap phase is shown to be equivalent to the (anisotropic) quantum electrodynamics in (2+1) space-time dimensions (QED$_3$). The role of Dirac fermions is played by the nodal BdG quasiparticles while the massless gauge field arises through unbinding of quantum vortex-antivortex degrees of freedom. A detailed derivation of this QED$_3$ theory is given and some of its main physical consequences are inferred for the pseudogap state. We focus on the properties of symmetric QED$_3$ and propose that inside the pairing protectorate it assumes the role reminiscent of that played by the Fermi liquid theory in conventional metals.

cond-mat.supr-con↗

arXiv2002

Gauge invariant fermion propagator in QED3

In a recent preprint [cond-mat/0204040] Khveshchenko questioned the validity of our computation of the gauge invariant fermion propagator in QED3, which we employed as an effective theory of high-T_c cuprate superconductors [cond-mat/0203333]. We take this opportunity to further clarify our procedure and to show that criticism voiced in the above preprint is unwarranted.

cond-mat.str-el↗

arXiv2002

Relativity Restored: Dirac Anisotropy in QED$_3$

We show that at long lengthscales and low energies and to leading order in 1/N expansion, the anisotropic QED in 2+1 dimensions renormalizes to an isotropic limit. Consequently, the (Euclidean) relativistic invariance of the theory is spontaneously restored at the isotropic critical point, characterized by the anomalous dimension exponent of the Dirac fermion propagator $η$. We find $η=16/3π^2 N$.

cond-mat.supr-con↗

arXiv2001

Chiral Symmetry Breaking and Phase Fluctuations in Cuprate Superconductors: A QED$_3$ Unified Theory of the Pseudogap State

A d-wave superconductor, its phase coherence progressively destroyed by unbinding of vortex-antivortex pairs, suffers an instability related to chiral symmetry breaking in two-flavor QED$_3$. The chiral manifold exhibits large degeneracy spanned by physical states acting as inherent ``competitors'' of d-wave superconductivity. Two of these states are associated with antiferromagnetic insulator and ``stripe'' phases, known to be stable in the pseudogap regime of cuprates near half-filling. The theory also predicts additional, yet unobserved state: a d+ip phase-incoherent superconductor.

cond-mat.supr-con↗

arXiv2001

Universal thermal conductivity in the vortex state of cuprate superconductors

We formulate an effective low energy theory for the fermionic excitations in d-wave superconductors in the presence of periodic vortex lattices. These can be modeled by an effective free Dirac Hamiltonian with renormalized velocities and possibly a small mass term. In the presence of random nonmagnetic impurities this will result in universal (i.e. field and disorder strength independent) thermal and spin conductivities with values different from those occurring in the Meissner state.

cond-mat.supr-con↗

arXiv2001

Quasiparticle Hall Transport of d-wave Superconductors in Vortex State

We present a theory of quasiparticle Hall transport in strongly type-II superconductors within their vortex state. We establish the existence of integer quantum spin Hall effect in clean unconventional $d_{x^2-y^2}$ superconductors in the vortex state from a general analysis of the Bogoliubov-de Gennes equation. The spin Hall conductivity $σ^s_{xy}$ is shown to be quantized in units of $\frac{\hbar}{8π}$. This result does not rest on linearization of the BdG equations around Dirac nodes and therefore includes inter-nodal physics in its entirety. In addition, this result holds for a generic inversion-symmetric lattice of vortices as long as the magnetic field $B$ satisfies $H_{c1} \ll B \ll H_{c2}$. We then derive the Wiedemann-Franz law for the spin and thermal Hall conductivity in the vortex state. In the limit of $T \to 0$, the thermal Hall conductivity satisfies $κ_{x y}=\frac{4π^2}{3}(\frac{k_B}{\hbar})^2 T σ^s_{xy}$. The transitions between different quantized values of $σ^s_{xy}$ as well as relation to conventional superconductors are discussed.

cond-mat.supr-con↗

arXiv2000

Quasiparticles and Vortices in Unconventional Superconductors

Quasiparticles in the vortex lattice of strongly type-II superconductors are investigated by means of a singular gauge transformation applied to the tight binding lattice Bogoliubov-de Gennes Hamiltonian. We present a detailed derivation of the gauge invariant effective low energy Hamiltonian for the quasiparticle-vortex system and show how the physics of the "Doppler shift" and "Berry phase" can be incorporated at the Hamiltonian level by working in the singular gauge. In particular, we show that the "Berry phase" effect manifests itself in the effective Hamiltonian through a half-flux Aharonov-Bohm scattering of quasiparticles off vortices and stress the important role that this effect plays in the quasiparticle dynamics. Full numerical solutions in the regime of intermediate fields H_c1<<B<<H_c2 are presented for model superconductors with s-, p- and d-wave symmetries and with square and triangular vortex lattices. For s- and p-wave cases we obtain low energy bound states in the core, in agreement with the existing results. For d-wave case only extended quasiparticle states exist. We investigate in detail the nature of these extended states and provide comparison to the previous results within linearized ``Dirac fermion'' model. We also investigate internodal interference effects when vortex and ionic lattices have high degree of commensurability and discuss various specific choices for the singular gauge transformation.

cond-mat.supr-con↗