arXiv subjects

Predrag Nikolic

Publications and source records attributed to Predrag Nikolic.

28 records · Page 2Linked to original sources

arXiv2011

Robust accidental nodes and zeroes and critical quasiparticle scaling in iron-based multiband superconductors

We study multigap superconductivity, with strong angular variations of one of the gaps, as appropriate for certain iron-based high-temperature superconductors. We solve the gap equations of this model and find that the nodes or zeroes in the gap function present at Tc - although purely accidental -- typically survive down to T=0. Based on this result, we investigate the line of quantum transitions at which gap zeroes first appear. The peculiar "zero-point" critical scaling emanating from this line dominates quasiparticle thermodynamics and transport properties over much of the phase diagram, and supplants more familiar forms of scaling associated with accidental nodes.

cond-mat.supr-con↗

arXiv2010

Superconductivity in Multi-orbital t-J1-J2 Model and its Implications for Iron Pnictides

Motivated by the bad metal behavior of the iron pnictides, we study a multi-orbital $\mathrm{t-J_1-J_2}$ model and investigate possible singlet superconducting pairings. Magnetic frustration by itself leads to a large degeneracy in the pairing states. The kinetic energy breaks this into a quasi-degeneracy among a reduced set of pairing states. For small electron and hole Fermi pockets, an $A_{1g}$ state dominates over the phase diagram but a $B_{1g}$ state has close-by energy. In addition to the nodeless $A_{1g}$ $s_{x^2y^2}$ channel, the nodal $A_{1g}$ $s_{x^2+y^2}$ and $B_{1g}$ $d_{x^2-y^2}$ channels are also competitive in the magnetically frustrated $J_1 \sim J_2$ parameter regime. An $A_{1g}+i B_{1g}$ state, which breaks time-reversal symmetry, occurs at low temperatures in part of the phase diagram. Implications for the experiments in the iron pnictides are discussed.

cond-mat.supr-con↗

arXiv2010

Pair density wave instability and Cooper pair insulators in gapped fermion systems

By analyzing simple models of fermions in lattice potentials we argue that the zero-temperature pairing instability of any ideal band-insulator occurs at a finite momentum. The resulting supersolid state is known as "pair density wave". The pairing momentum at the onset of instability is generally incommensurate as a result of phase-space restrictions and relative strengths of interband and intraband pairing. However, commensurate pairing occurs in the strong-coupling limit and becomes a Cooper-channel analogue of the Halperin-Rice exciton condensation instability in indirect bandgap semiconductors. The exceptional sensitivity of incommensurate pairing to quantum fluctuations can lead to a strongly-correlated insulating regime and a non-BCS transition, even in the case of weak coupling as shown by an exact renormalization group analysis.

cond-mat.supr-con↗

arXiv2008

Pairing instability driven by macroscopically degenerate collective modes in two-dimensional rotating fermion liquids near unitarity

Fermionic superfluids can undergo phase transitions into different kinds of normal regimes, loosely characterized by whether Cooper pairs remain locally stable. If the normal phase retains strong pairing fluctuations, it behaves like a liquid of vortices, which has been observed in cuprate superconductors. We argue that analogous strongly correlated normal states exist in two-dimensional neutral fermion liquids near unitarity, where superfluid is destroyed by fast rotation. These states have non-universal properties, and if they develop as distinct thermodynamic phases they can be characterized as quantum Hall states of Cooper pairs. The formal analysis is based on a model with SP(2N) symmetry that describes the quantum critical region in the vicinity of a broad Feshbach resonance. We explore the pairing phase diagram and demonstrate that the considered model has macroscopically degenerate bosonic modes in the normal phase, to all orders in 1/N. It takes finite-range interactions to lift this degeneracy, making the Abrikosov flux lattice of the superfluid particularly susceptible to quantum melting.

cond-mat.supr-con↗

arXiv2007

Superfluid-insulator transitions of the Fermi gas with near-unitary interactions in a periodic potential

We consider spin-1/2 fermions of mass m with interactions near the unitary limit. In an applied periodic potential of amplitude V and period a, and with a density of an even integer number of fermions per unit cell, there is a second-order quantum phase transition between superfluid and insulating ground states at a critical V=Vc. We compute the universal ratio Vc m a^2 / h^2 at N=infinity in a model with Sp(2N) spin symmetry. The insulator interpolates between a band insulator of fermions and a Mott insulator of fermion pairs. We discuss implications for recent experiments.

cond-mat.supr-con↗

arXiv2007

Renormalization group fixed points, universal phase diagram, and 1/N expansion for quantum liquids with interactions near the unitarity limit

It has long been known that particles with short-range repulsive interactions in spatial dimension d=1 form universal quantum liquids in the low density limit: all properties can be related to those of the spinless free Fermi gas. Previous renormalization group (RG) analyses demonstrated that this universality is described by an RG fixed point, infrared stable for d<2, of the zero density gas. We show that for d>2 the same fixed point describes the universal properties of particles with short-range attractive interactions near a Feshbach resonance; the fixed point is now infrared unstable, and the relevant perturbation is the detuning of the resonance. Some exponents are determined exactly, and the same expansion in powers of (d-2) applies for scaling functions for d<2 and d>2. A separate exact RG analysis of a field theory of the particles coupled to `molecules' finds an alternative description of the same fixed point, with identical exponents; this approach yields a (4-d) expansion which agrees with the recent results of Nishida and Son (cond-mat/0604500). The existence of the RG fixed point implies a universal phase diagram as a function of density, temperature, population imbalance, and detuning; in particular, this applies to the BEC-BCS crossover of fermions with s-wave pairing. Our results open the way towards computation of these universal properties using the standard field-theoretic techniques of critical phenomena, along with a systematic analysis of corrections to universality. We also propose a 1/N expansion (based upon models with Sp(2N) symmetry) of the fixed point and its vicinity, and use it to obtain results for the phase diagram.

cond-mat.supr-con↗

arXiv2006

Electronic states near a quantum fluctuating point vortex in a d-wave superconductor: Dirac fermion theory

We introduce a simple model of the low energy electronic states in the vicinity of a vortex undergoing quantum zero-point motion in a d-wave superconductor. The vortex is treated as a point flux tube, carrying pi-flux of an auxiliary U(1) gauge field, which executes simple harmonic motion in a pinning potential. The nodal Bogoliubov quasiparticles are represented by Dirac fermions with unit U(1) gauge charge. The energy dependence of the local density of electronic states (LDOS) at the vortex center has no zero bias peak; instead, small satellite features appear, driven by transitions between different vortex eigenmodes. These results are qualitatively consistent with scanning tunneling microscopy measurements of the sub-gap LDOS in cuprate superconductors. Furthermore, as argued in L. Balents et al., Phys.Rev.B 71, 144508 (2005), the zero-point vortex motion also leads naturally to the observed periodic modulations in the spatial dependence of the sub-gap LDOS.

cond-mat.supr-con↗

arXiv2006

Low energy theory of a single vortex and electronic quasiparticles in a d-wave superconductor

We highlight the properties of a simple model (contained in our recent work) of the quantum dynamics of a single point vortex interacting with the nodal fermionic quasiparticles of a d-wave superconductor. We describe the renormalization of the vortex motion by the quasiparticles: at T=0, the quasiparticles renormalize the vortex mass and introduce only a weak sub-Ohmic damping. Ohmic (or `Bardeen-Stephen' damping) appears at T>0, with the damping co-efficient vanishing ~ T^2 with a universal prefactor. Conversely, quantum fluctuations of the vortex renormalize the quasiparticle spectrum. A point vortex oscillating in a harmonic pinning potential has no zero-bias peak in the electronic local density of states (LDOS), but has small satellite features at an energy determined by the pinning potential. These are proposed as the origin of sub-gap LDOS peaks observed in scanning tunneling microscopic studies of the LDOS near a vortex.

cond-mat.str-el↗

arXiv2005

Effective action for vortex dynamics in clean d-wave superconductors

We describe the influence of the gapless, nodal, fermionic quasiparticles of a two-dimensional d-wave superconductor on the motion of vortices. A continuum, functional formalism is used to obtain the effective vortex action, after the fermions have been integrated out. At zero temperture (T), the leading terms in the vortex action retain their original form, with only a finite renormalization of the vortex effective mass from the fermions. A universal "sub-ohmic" damping of the vortex motion is also found. At T>0, we find a Bardeen-Stephen viscous drag term, with a universal co-efficient which vanishes as ~T^2. We present a simple scaling interpretation of our results, in which quantum-critical Dirac fermions respond to a moving point singularity. Our results appear to differ from those of the semiclassical theory, which obtains more singular corrections to a vortex mass appearing in transport equations.

cond-mat.str-el↗

arXiv2004

Disordered, Spin Liquid and Valence-Bond Ordered Phases of the Kagome Lattice Quantum Ising Models With Transverse Field and XXZ Dynamics

General conditions in which disordered, spin liquid, and valence-bond ordered phases occur in quantum Ising antiferromagnets are studied using the prototype Kagome lattice spin models. A range of quantum dynamical processes in the Ising model, with and without total Ising spin conserved, are analytically shown to yield all three characteristic quantum paramagnetic phases in the Kagome system. Special emphasis is given to the XXZ model that can be sensibly compared to the Kagome lattice Heisenberg antiferromagnet. It is explicitly demonstrated that the total-spin-conserving dynamics can yield a resonant valence bond (RVB) liquid phase with very short-ranged correlations, but also a valence-bond ordered phase compatible with the one proposed to explain the seemingly gapless singlet states of the Heisenberg antiferromagnet on the Kagome lattice. Likely consequences for generic spin models are discussed. The analysis combines compact U(1) gauge theory, duality transformations, lattice-field-theoretical methods, and variational approach.

cond-mat.str-el↗