arXiv subjects

L. B. Ioffe

Publications and source records attributed to L. B. Ioffe.

85 records · Page 5Linked to original sources

arXiv1995

Theory of the Spin Gap in High Temperature Superconductors

We analyze pairing in two dimensional spin liquids. We argue that interplane pairing enhanced by magnetic correlations is the most plausible explanation of the spin gap phenomenon observed in underdoped cuprates. The details of the pairing theory depend on the in-plane antiferromagnetic correlations. We consider two models: $2p_F$ correlations induced by a strong gauge field interaction and undamped spin waves. We estimate the pairing temperature $T_s$ and the angular dependence of the gap function and discuss physical consequences.

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arXiv1995

The $\bf T=0$ $\bf 2k_F$ density wave phase transition in a two dimensional Fermi liquid is first order

We study $T=0$ spin density wave transitions in two dimensional Fermi liquids in which the ordering wavevector $\bf Q$ is such that the tangents to the Fermi line at the points connected by $\bf Q$ are parallel (e.g. $Q=2p_F$ in a system with a circular Fermi line) and the Fermi line is not flat. We show that the transition is first order if the ordering wave vector $\bf Q$ is not commensurate with a reciprocal lattice vector, $\bf G$, i.e. ${\bf Q} \neq {\bf G}/2$. If $\bf Q$ is close to ${\bf G}/2$ the transition is weakly first order and an intermediate scaling regime exists; in this regime the $2p_F$ susceptibility and observables such as the NMR rates $T_1$ and $T_2$ have scaling forms which we determine.

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arXiv1995

Magnetic Properties of High Temperature Superconductors: a Spin Liquid Approach. Talk given at Stanford Spectroscopy Meeting, 1995.

We argue that strongly correlated two dimensional electrons form a spin liquid in some regimes of density and temperature and give the theory of the magnetic properties of this spin liquid using the representation in terms of femions interacting with a gauge field. This state is characterized by anomalous power law spin correlations and discuss the implications of these correlations for the temperature dependence of NMR relaxation rates $1/T_1$ and $1/T_2$ and for the uniform susceptibility. We also discuss the transition from the spin liquid to antiferromagnet and the critical behavior of these properties at the transition. We compare these theoretical results with the data on high $T_c$ superconductors. Finally, we discuss the formation of the spin gap due to the spin exhange interaction between adjacent layers in bilayered materials.

cond-mat↗

arXiv1995

Spin Gaps in High Temperature Superconductors

The phenomenology and theory of spin gap effects in high temperature superconductors is summarized. It is argued that the spin gap behavior can only be explained by a model of charge 0 spin 1/2 fermions which become paired into singlets and that there are both theoretical and experimental reasons for believing that the pairing is greatly enhanced in the bilayer structure of the $YBa_2Cu_3O_{6+x}$ system. This article will appear in the Proceedings of the Stanford Conference on Spectroscopies in Novel Superconductors. To obtain postscript files containing the figures send mail to millis@physics.att.com.

cond-mat↗

arXiv1995

Possible Glassiness in a Periodic Long-Range Josephson Array

We present an analytic study of a periodic Josephson array with long-range interactions in a transverse magnetic field. We find that this system exhibits a first-order transition into a phase characterized by an extensive number of states separated by barriers that scale with the system size; the associated discontinuity is small in the limit of weak applied field, thus permitting an explicit analysis in this regime.

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arXiv1995

On the spin density wave transition in a two dimensional spin liquid

Strongly correlated two dimensional electrons are believed to form a spin liquid in some regimes of density and temperature. As the density is varied, one expects a transition from this spin liquid state to a spin density wave antiferromagnetic state. In this paper we show that it is self-consistent to assume that this transition is second order and, on this assumption, determine the critical behavior of the $2p_F$ susceptibility, the NMR rates $T_1$ and $T_2$ and the uniform susceptibility. We compare our results to data on high $T_c$ materials.

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arXiv1994

Non local screening in a vortex line liquid

We show that the recent experiments \cite{safar} reporting the onset of the non-local conductivity in the vortex state of $YBaCuO$ single crystals indicate the presence of a new liquid phase of vortices. This phase is intermediate between the normal metal and the Abrikosov lattice. We use the mapping of the vortex problem to the problem of bose liquid to determine theoretically the properties of the proposed vortex liquid phase and compare them with the data.

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arXiv1994

On the Critical Behavior of the Uniform Susceptibility of a Fermi Liquid Near an Antiferromagnetic Transition with Dynamic Exponent $ z = 2$

We compute the leading behavior of the uniform magnetic susceptibility, $χ$, of a Fermi liquid near an antiferromagnetic transition with dynamic exponent $z=2$. Our calculation clarifies the role of triangular ``anomaly'' graphs in the theory and justifies the effective action used in previous work \cite{Hertz}. We find that at the $z=2$ critical point of a two dimensional material, $lim_{q \rightarrow 0} χ(q,0) = χ_0 - D T$ with $χ_0$ and $D$ nonuniversal constants. For reasonable band structures we find that in a weak coupling approximation $D$ is small and positive. Our result suggests that the behavior observed in the quantum critical regime of underdoped high-$T_c$ superconductors are difficult to explain in a $z=2$ theory.

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arXiv1994

On the low energy properies of fermions with singular interactions

We calculate the fermion Green function and particle-hole susceptibilities for a degenerate two-dimensional fermion system with a singular gauge interaction. We show that this is a strong coupling problem, with no small parameter other than the fermion spin degeneracy, N. We consider two interactions, one arising in the context of the $t-J$ model and the other in the theory of half-filled Landau level. For the fermion self energy we show in contrast to previous claims that the qualitative behavior found in the leading order of perturbation theory is preserved to all orders in the interaction. The susceptibility $χ_Q$ at a general wavevector $\bf{Q} \neq 2\bf{p_F}$ retains the fermi-liquid form. However the $2p_F$ susceptibility $χ_{2p_F}$ either diverges as $T -> 0$ or remains finite but with nonanalytic wavevector, frequency and temperature dependence. We express our results in the language of recently discussed scaling theories, give the fixed-point action, and show that at this fixed point the fermion-gauge-field interaction is marginal in $d=2$, but irrelevant at low energies in $d \ge 2$.

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arXiv1994

On the theory of diamagnetism in granular superconductors

We study a highly disordered network of superconducting granules linked by weak Josephson junctions in magnetic field and develop a mean field theory for this problem. The diamagnetic response to a slow {\it variations} of magnetic field is found to be analogous to the response of a type-II superconductor with extremely strong pinning. We calculate an effective penetration depth $λ_g$ and critical current $j_c$ and find that both $λ_g^{-1}$ and $j_c$ are non-zero but are strongly suppressed by frustration.

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arXiv1994

Effective lowering of the dimensionality in strongly correlated two dimensional electron gas

We study a system of fermions interacting with a gauge field which can be used to describe either spin liquid or $ν=1/2$ Quantum Hall state. We propose a generalized model with a dimensionless parameter $N$. We evaluate the properties of the model in both limits $N \gg 1$ and $N \ll 1$ and deduce the properties of the model in the most physically intersting case of $N=1,2$. At $N \ll 1$ the motion of the fermions becomes one dimensional. This allows us to obtain the fermion Green function and response functions applying bozonization method in this limit.

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arXiv1993

On the theory of the spin gap in bilayer cuprates

We formulate and solve a model of two planes of antiferromagnetically correlated electrons coupled together by a weak antiferromagnetic interaction of strength $λ$. We show that in-plane antiferromagnetic correlations dramatically enhance the pairing effect of the interplane interaction. For the case where the in-plane correlation length $κ^{-1} \sim T^{-1/2}$, we find that the interaction $λ$ leads to spin pairing at a temperature $T^{*} \sim λ$, much higher than the usual BCS result $exp(-J/λ)$. We suggest that this is a possible explanation of the spin gap effects observed below $T^{*} \sim 150K$ in $YBa_2Cu_3O_{6.6}$.

cond-mat↗

arXiv1993

Finite-Temperature Transition into a Power-Law Spin Phase with an Extensive Zero-Point Entropy

We introduce an $xy$ generalization of the frustrated Ising model on a triangular lattice. The presence of continuous degrees of freedom stabilizes a {\em finite-temperature} spin state with {\em power-law} discrete spin correlations and an extensive zero-point entropy. In this phase, the unquenched degrees of freedom can be described by a fluctuating surface with logarithmic height correlations. Finite-size Monte Carlo simulations have been used to characterize the exponents of the transition and the dynamics of the low-temperature phase.

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