arXiv subjects

Eugene Demler

Publications and source records attributed to Eugene Demler.

At least 361 records · Page 20Linked to original sources

arXiv2002

Novel Josephson effects between multi-gap and single-gap superconductors

Multi-gap superconductors can exhibit qualitatively new phenomena due to existence of multiple order parameters. Repulsive electronic interactions may give rise to a phase difference of $π$ between the phases of the order parameters. Collective modes due to the oscillation of the relative phases of these order parameters are also possible. Here we show that both these phenomena are observable in Josephson junctions between a single-gap and a multi-gap superconductor. In particular, a non-monotonic temperature dependence of the Josephson current through the junction reveals the existence of the $π$ phase differences in the multi-gap superconductor. This mechanism may be relevant for understanding several experiments on the Josephson junctions with unconventional superconductors. We also discuss how the presence of the collective mode resonantly enhances the DC Josephson current when the voltage across the junction matches the mode frequency. We suggest that our results may apply to MgB$_2$, 2H-NbSe$_2$, spin ladder and bilayer cuprates.

cond-mat.supr-con↗

arXiv2001

Fractionalization patterns in strongly correlated electron systems: Spin-charge separation and beyond

We discuss possible patterns of electron fractionalization in strongly interacting electron systems. A popular possibility is one in which the charge of the electron has been liberated from its Fermi statistics. Such a fractionalized phase contains in it the seed of superconductivity. Another possibility occurs when the spin of the electron, rather than its charge, is liberated from its Fermi statistics. Such a phase contains in it the seed of magnetism, rather than superconductivity. We consider models in which both of these phases occur and study possible phase transitions between them. We describe other fractionalized phases, distinct from these, in which fractions of the electron themselves fractionalize, and discuss the topological characterization of such phases. These ideas are illustrated with specific models of p-wave superconductors, Kondo lattices, and coexistence between d-wave superconductivity and antiferromagnetism.

cond-mat.str-el↗

arXiv2001

Magnetic field tuning of charge and spin order in the cuprate superconductors

Recent neutron scattering, nuclear magnetic resonance, and scanning tunneling microscopy experiments have yielded valuable new information on the interplay between charge and spin density wave order and superconductivity in the cuprate superconductors, by using an applied perpendicular magnetic field to tune the ground state properties. We compare the results of these experiments with the predictions of a theory which assumed that the ordinary superconductor was proximate to a quantum transition to a superconductor with co-existing spin/charge density wave order.

cond-mat.str-el↗

arXiv2001

Fermions and Bosons in Superconducting Amorphous Wires

We discuss the destruction of superconductivity in quasi-one-dimensional systems due to the interplay between disorder and Coulomb repulsion. We argue that to understand the behavior of the system one has to study both fermionic and bosonic mechanisms of suppression of superconductivity. The former describes reduction in the mean field critical temperature $T_c$, while the latter refers to thermal and quantum fluctuations in the order parameter. A change in parameters such as wire width and disorder strength significantly affects both mechanisms.

cond-mat.supr-con↗

arXiv2001

Spin ordering quantum transitions of superconductors in a magnetic field

We argue that recent neutron scattering measurements by Lake et. al. (Science 291, 1759 (2001)) of the spin excitation spectrum of LSCO in a magnetic field can be understood in terms of proximity to a phase with co-existing superconductivity and spin density wave order. We present a general theory for such quantum transitions, and argue that their low energy spin fluctuations are controlled by a singular correction from the superflow kinetic energy, acting in the region outside the vortex cores. We propose numerous experimental tests of our theory.

cond-mat.supr-con↗

arXiv2001

Spinor Bosonic Atoms in Optical Lattices: Symmetry Breaking and Fractionalization

We study superfluid and Mott insulator phases of cold spin-1 Bose atoms with antiferromagnetic interactions in an optical lattice, including a usual polar condensate phase, a condensate of singlet pairs, a crystal spin nematic phase, and a spin singlet crystal phase. We suggest a possibility of exotic fractionalized phases of spinor BEC and discuss them in the language of topological defect condensation and $Z_2$ lattice gauge theory.

cond-mat.stat-mech↗

arXiv2000

Bilayer Quantum Hall States at $ν=1$ and Coulomb Drag

We consider a number of strongly-correlated quantum Hall states which are likely to be realized in bilayer quantum Hall systems at total Landau level filling fraction ${ν_T}=1$. One state, the $(3,3,-1)$ state, can occur as an instability of a compressible state in the large $d/l_B$ limit, where $d$ and $l_B$ are the interlayer distance and magnetic length, respectively. This state has a hierarchical descendent which is interlayer coherent. Another interlayer coherent state, which is expected in the small $d/l_B$ limit is the well-known Halperin $(1,1,1)$ state. Using the concept of composite fermion pairing, we discuss the wavefunctions which describe these states. We construct a phase diagram using the Chern-Simons Landau-Ginzburg theory and discuss the transitions between the various phases. We propose that the longitudinal and Hall drag resistivities can be used together with interlayer tunneling to experimentally distinguish these different quantum Hall states. Our work indicates the bilayer ${ν_T}=1$ quantum Hall phase diagram to be considerably richer than that assumed so far in the literature.

cond-mat.mes-hall↗

arXiv2000

One-dimensional Surface Bound States in d-wave Superconductors

We present an {\it exact} quantum theory for the bound states in the vicinity of an edge or a line of impurities in a $d_{x^2-y^2}$ superconductor. For a (110)-surface we show that a finite dispersion of the one dimensional band of bound states leads to a two peak structure in the density of states (DOS). We study the effect of an applied magnetic field and a subdominant $id_{xy}$ order parameter on the DOS and discuss the implications of our results for tunneling experiments.

cond-mat.supr-con↗

arXiv2000

Bi-layer Paired Quantum Hall States and Coulomb Drag

We consider three states which are likely to be realized in bi-layer quantum Hall systems at total Landau level filling fraction ${ν_T}=1$. Two of these states may be understood as paired states. One can occur as an instability of a compressible state in the large $d/l_B$ limit, where $d$ and $l_B$ are the interlayer distance and magnetic length. The other is the $(1,1,1)$ state, which is expected in the small $d/l_B$ limit. We propose that the longitudinal and Hall drag resistivities can be used to distinguish these different quantum Hall states. The drag resistivities may be computed in a straightforward manner using the paired state formulation.

cond-mat.mes-hall↗

arXiv2000

Bilayer Quantum Hall Systems: Spin-Pseudospin Symmetry Breaking and Quantum Phase Transitions

We discuss and review recent advances in our understaning of quantum Hall systems where additional quantum numbers associated with spin and/or layer (pseudospin) indices play crucial roles in creating exotic quantum phases. Among the novel quantum phases we discuss are the recently discovered canted antiferromagnetic phase, the spontaneous interlayer coherent phase, and various spin Bose glass phases. We describe the theoretical models used in studying these novel phases and the various experimental techniques being used to search for these phases. Both zero temperature quantum phase transitions and finite temperature phase transitions are discussed. Emphasis in this article is on the recent developments in novel quantum phases and quantum phase transitions in bilayer quantum Hall systems where nontrivial magnetic ground states associated with spontaneous spin symmetry breaking play central role.

cond-mat.mes-hall↗

arXiv2000

Bilayer Coherent and Quantum Hall Phases: Duality and Quantum Disorder

We consider a fully spin-polarized quantum Hall system with no interlayer tunneling at total filling factor $ν=1/k$ (where $k$ is an odd integer) using the Chern-Simons-Ginzburg-Landau theory. Exploiting particle-vortex duality and the concept of quantum disordering, we find a large number of possible compressible and incompressible ground states, some of which may have relevance to recent experiments of Spielman {\it et.al.}. We find interlayer coherent compressible states without Hall quantization and interlayer-incoherent incompressible with Hall quantization in addition to the usual $(k,k,k)$ Halperin states, which are both interlayer-coherent and incompressible.

cond-mat.mes-hall↗

arXiv1999

Spin symmetry breaking in bilayer quantum Hall systems

Based on the construction of generalized Halperin wave functions, we predict the possible existence of a large class of broken spin symmetry states in bilayer quantum Hall structures, generalizing the recently suggested canted antiferromgnetic phase to many fractional fillings. We develop the appropriate Chern-Simons theory, and establish explicitly that the low-lying neutral excitation is a Goldstone mode and that the charged excitations are bimerons with continuously tunable (through the canted antiferromagnetic order parameter) electric charge on the individual merons.

cond-mat.mes-hall↗

arXiv1999

Resonant neutron scattering on the high Tc cuprates and pi and eta excitation of the t-J and Hubbard models

We review the explanation for resonant neutron scattering experiments in $YBa_2Cu_3O_{6+δ}$ and $Bi_2 Sr_2 Ca_2 O_{8+δ}$ materials from the point of view of triplet excitation in the particle-particle channel, the pi excitation. Relation of these resonances to the superconducting condensation energy and their role in stabilizing the superconducting state is discussed. Due to the superconducting fluctuations, the pi resonance may appear as a broad peak above Tc. Analogue problem to the pi excitation for the case of s-wave pairing, the eta excitation, is considered in the strong coupling limit with an emphasis on the resonance precursors in a state with no long range order.

cond-mat.str-el↗

arXiv1999

Quantitative Determination of a Microscopic Mechanism of High Tc Superconductivity

Within the t-J model of copper-oxides, the condensation energy can be related to the change in the dynamical spin structure factor between the superconducting and the normal states. By analyzing previous experimental data, we show that the change associated with the resonant neutron scattering peak found in $YBa_2Cu_3O_7$ can quantitatively account for the condensation energy. We argue that this analysis suggests a microscopic mechanism for high Tc superconductivity.

cond-mat.str-el↗

arXiv1998

Spin Bose Glass Phase in Bilayer Quantum Hall Systems at $ν=2$

We develop an effective spin theory to describe magnetic properties of the $ν=2$ Quantum Hall bilayer systems. In the absence of disorder this theory gives quantitative agreement with the results of microscopic Hartree-Fock calculations, and for finite disorder it predicts the existence of a novel spin Bose glass phase. The Bose glass is characterized by the presence of domains of canted antiferromagnetic phase with zero average antiferromagnetic order and short range mean antiferromagnetic correlations. It has infinite antiferromagnetic transverse susceptibility, finite longitudinal spin susceptibility and specific heat linear in temperature. Transition from the canted antiferromagnet phase to the spin Bose glass phase is characterized by a universal value of the longitudinal spin conductance.

cond-mat.mes-hall↗

arXiv1998

Non-Abelian Holonomy of BCS and SDW Quasi-particles

In this work we investigate properties of fermions in the SO(5) theory of high Tc superconductivity. We show that the adiabatic time evolution of a SO(5) superspin vector leads to a non-Abelian SU(2) holonomy of the SO(5) spinor states. Physically, this non-trivial holonomy arises from the non-zero overlap between the SDW and BCS quasi-particle states. While the usual Berry's phase of a SO(3) spinor is described by a Dirac magnetic monopole at the degeneracy point, the non-Abelian holonomy of a SO(5) spinor is described by a Yang monopole at the degeneracy point, and is deeply related to the existence of the second Hopf map from $S^7$ to $S^4$. We conclude this work by extending the bosonic SO(5) nonlinear sigma model to include the fermionic states around the gap nodes as 4 component Dirac fermions coupled to SU(2) gauge fields in 2+1 dimensions.

cond-mat.str-el↗

arXiv1997

Pi excitation of the t-J model

In this paper, we present analytical and numerical calculations of the pi resonance in the t-J model. We show in detail how the pi resonance in the particle-particle channel couples to and appears in the dynamical spin correlation function in a superconducting state. The contribution of the pi resonance to the spin excitation spectrum can be estimated from general model-independent sum rules, and it agrees with our detailed calculations. The results are in overall agreement with the exact diagonalization studies of the t-J model. Earlier calculations predicted the correct doping dependence of the neutron resonance peak in the YBCO superconductor, and in this paper detailed energy and momentum dependence of the spin correlation function is presented. The microscopic equations of motion obtained within current formalism agree with that of the SO(5) nonlinear sigma model, where the pi resonance is interpreted as a pseudo Goldstone mode of the spontaneous SO(5) symmetry breaking.

cond-mat.supr-con↗

arXiv1997

Microscopic Electron Models with Exact SO(5) Symmetry

We construct a class of microscopic electron models with exact SO(5) symmetry between antiferromagnetic and d-wave superconducting ground states. There is an exact one-to-one correspondence between both single-particle and collective excitations in both phases. SO(5) symmetry breaking terms can be introduced and classified according to irreducible representations of the exact SO(5) algebra. The resulting phase diagram and collective modes are identical to that of the SO(5) nonlinear sigma model.

cond-mat.supr-con↗