arXiv subjects

P. Coleman

Publications and source records attributed to P. Coleman.

At least 109 records · Page 6Linked to original sources

arXiv2000

Onset of antiferromagnetism in heavy fermion metals

There are two views of antiferromagnets. The first proceeds from atomic physics, which predicts that atoms with unpaired electrons develop magnetic moments. In a solid, the coupling between moments on nearby ions then yields antiferromagnetic order at low temperatures. The second, based on the physics of electron fluids or 'Fermi liquids', states that Coulomb interactions can drive the fluid to adopt a more stable configuration by developing a spin density wave. It is presently unknown which view is appropriate at a `quantum critical point', where the antiferromagnetic transition temperature vanishes. Here we describe an atomically local contribution to the magnetic correlations which develops in the metal CeCu$_{6-x}$Au$_{x}$ at the critical gold concentration ($x_c=0.1$) where the magnetic ordering temperature is tuned to zero. This contribution implies that a Fermi-liquid destroying spin-localizing transition, unanticipated for the spin density wave description, coincides with the antiferromagnetic quantum critical point.

cond-mat.str-el↗

arXiv2000

Strong Coupling Approach to the Supersymmetric Kondo Model

We carry out the strong coupling expansion for the SU(N) Kondo model where the impurity spin is represented by a L-shaped Young tableau. Using second order perturbation theory around the strong coupling fixed point it is shown that when the antisymmetric component of the Young-tableau contains more than N/2 entries, the strong-coupling fixed point becomes unstable to a two-stage Kondo effect. By comparing the strong coupling results obtained here with the result using a supersymmetric large N expansion, we are also able to confirm the validity of the the supersymmetric formalism for mixed symmetry Kondo models.

cond-mat.str-el↗

arXiv1999

Gap-anisotropic model for the narrow-gap Kondo insulators

A theory is presented which accounts for the dynamical generation of a hybridization gap with nodes in the Kondo insulating materials $CeNiSn$ and $CeRhSb$. We show that Hunds interactions acting on virtual $4f^2$ configurations of the cerium ion can act to dynamically select the shape of the cerium ion by generating a Weiss field which couples to the shape of the ion. In low symmetry crystals where the external crystal fields are negligible, this process selects a nodal Kondo semimetal state as the lowest energy configuration.

cond-mat.str-el↗

arXiv1999

Supersymmetric spin operators

We develop a supersymmetric representation of spin operators which unifies the Schwinger and Abrikosov representations of SU(N) spin operators, allowing a second-quantized treatment of representations of the SU(N) group with both symmetric and antisymmetric character. By applying this to the SU(N) Kondo model, we show that it is possible to develop a controlled treatment of both Magnetism and the Kondo effect within a single large N expansion.

cond-mat.str-el↗

arXiv1999

Hidden Order in $URu_2Si_2$

We review current attempts to characterize the underlying nature of the hidden order in $URu_2Si_2$. A wide variety of experiments point to the existence of two order parameters: a large primary order parameter of unknown character which co-exists with secondary antiferromagnetic order. Current theories can be divided into two groups determined by whether or not the primary order parameter breaks time-reversal symmetry. We propose a series of experiments designed to test the time-reversal nature of the underlying primary order in $URu_2Si_2$ and to characterize its local single-ion physics.

cond-mat.str-el↗

arXiv1999

Bound-State Instability of the Chiral Luttinger Liquid in One-Dimension

We have developed a new boot-strap method for solving a class of interacting one-dimensional chiral fermions. The conventional model for interacting right-moving electrons with spin has an SO(4) symmetry, and can be written as four interacting Majorana fermions, each with the same velocity. We have found a method for solving some cases when the velocities of these Majorana fermions are no longer equal. We demonstrate in some detail the remarkable result that corrections to the non skeleton self-energy identically vanish for these models, and this enables us to solve them exactly. For the cases where the model can be solved by bosonization, our method can be explicitly checked. However, we are also able to solve some new cases where the excitation spectrum differs qualitatively from a Luttinger liquid. Of particular interest, is the so-called SO(3) model, where a triplet of Majorana fermions moving at one velocity, interact with a single Majorana fermion moving at another velocity. We show using our method, that a sharp bound (or anti-bound) state splits off from the original Luttinger liquid continuum, cutting off the X-ray singularity to form a broad incoherent excitation with a lifetime that grows linearly with frequency.

cond-mat.str-el↗

arXiv1999

Theories of non-Fermi liquid behavior in heavy fermions

I review our incomplete understanding of non-Fermi liquid behavior in heavy fermion systems at a quantum critical point. General considerations suggest that critical antiferromagnetic fluctuations do not destroy the Fermi surface by scattering the heavy electrons- but by actually breaking up the internal structure of the heavy fermion. I contrast the weak, and strong-coupling view of the quantum phase transition, emphasizing puzzles and questions that recent experiments raise.

cond-mat.str-el↗

arXiv1999

Breakdown of the Chiral Luttinger Liquid in One-Dimension

We have developed a fermionic boot-strap method to solve a class of chiral one dimensional fermion model which cannot be tackled by bosonization. Using this scheme, we show that Luttinger liquid behavior in a gas of four interacting chiral Majorana fermions is highly sensitive to the velocity degeneracy. Upon changing the velocity of a one chiral fermion, a sharp bound (or anti-bound) state splits off from the original Luttinger liquid continuum, cutting off the X-ray singularity to form a broad incoherent excitation with a lifetime that grows linearly with frequency.

cond-mat.str-el↗

arXiv1998

Co-operative Kondo Effect in the two-channel Kondo Lattice

We discuss the possibility of a co-operative Kondo effect driven by channel interference in a Kondo lattice where local moments are coupled to a single Fermi sea via two orthogonal scattering channels. In this situation, the channel quantum number is not conserved. We argue that the absence of channel conservation causes the Kondo effect in the two channels to constructively interfere, giving rise to a superconducting condensate of composite pairs, formed between the local moments and the conduction electrons. Our arguments are based on the observation that a heavy Fermi surface gives rise to zero modes for Kondo singlets to fluctuate between screening channels of different symmetry, producing a divergent composite pair susceptibility. Secondary screening channels couple to these divergent fluctuations, promoting an instability into a state with long-range composite order. We present detailed a detailed mean-field theory for this superconducting phase, and discuss the possible implications for heavy fermion physics.

cond-mat.str-el↗

arXiv1998

Scaling of magnetic fluctuations near a quantum phase transition

We use inelastic neutron scattering to measure the magnetic fluctuations in a single crystal of the heavy fermion alloy CeCu_5.9Au_0.1 close to the antiferromagnetic quantum critical point. The energy and temperature-dependent spectra obey (E/T) scaling at Q near (1,0,0). The neutron data and earlier bulk susceptibility are consistent with the form 1/X ~ f(Q)+(-iE+bT)^a, with an anomalous exponent a=0.8. We confirm the earlier observation of quasi-low dimensionality and show how both the magnetic fluctuations and the thermodynamics can be understood in terms of a quantum Lifshitz point.

cond-mat.str-el↗

arXiv1998

Marginal Fermi Liquid in a lattice of three-body bound-states

We study a lattice model for Marginal Fermi liquid behavior, involving a gas of electrons coupled to a dense lattice of three-body bound-states. The bound-states change the phase space for electron-electron scattering and induce a marginal self-energy amongst the electron gas. When the three-body bound-states are weakly coupled to the electron gas, there is a substantial window for marginal Fermi liquid behavior. In this regime, the model displays the presence of two relaxation times, one linear, one quadratic in the temperature. At low temperatures the bound-states develop coherence leading to a cross-over to conventional Fermi liquid behavior. At strong-coupling, marginal Fermi liquid behavior is pre-empted by a pairing or magnetic instability, and it is not possible to produce a linear scattering rate comparable with the temperature. We discuss the low temperature instabilities of this model and compare it to the Hubbard model at half-filling.

cond-mat.str-el↗

arXiv1997

Co-operative Two-Channel Kondo Effect

We discuss how the properties of a single-channel Kondo lattice model are modified by additional screening channels. Contrary to current wisdom, additional screening channels appear to constitute a relevant perturbation which destabilizes the Fermi liquid. When a heavy Fermi surface develops, it generates zero modes for Kondo singlets to fluctuate between screening channels of different symmetry, producing a divergent composite pair susceptibility. Additional screening channels couple to these divergent fluctuations, promoting an instability into a state with long-range composite order.

cond-mat.str-el↗

arXiv1997

Local Moments in an Interacting Environment

We discuss how local moment physics is modified by the presence of interactions in the conduction sea. Interactions in the conduction sea are shown to open up new symmetry channels for the exchange of spin with the localized moment. We illustrate this conclusion in the strong-coupling limit by carrying out a Schrieffer Wolff transformation for a local moment in an interacting electron sea, and show that these corrections become very severe in the approach to a Mott transition. As an example, we show how the Zhang Rice reduction of a two-band model is modified by these new effects.

cond-mat.str-el↗

arXiv1996

Reflections on the One Dimensional Realization of Odd-Frequency Pairing

We discuss the odd-frequency pairing correlations discovered by Zachar, Kivelson and Emery (ZKE) in a one dimensional Kondo lattice. A lattice model that realizes the continuum theory of ZKE is introduced and the correlations it gives rise to are identified as odd-frequency singlet pairing. The excitation spectrum is found to contain a spin gap, and a much lower energy band of spinless excitations. We discuss how the power-law correlations realized in the ZKE model evolve into true long-range order when Kondo chains are weakly coupled together and tentatively suggest a way in which the higher dimensional model can be treated using mean-field theory.

cond-mat.str-el↗

arXiv1996

How should we interpret the two transport relaxation times in the cuprates ?

We observe that the appearance of two transport relaxation times in the various transport coefficients of cuprate metals may be understood in terms of scattering processes that discriminate between currents that are even, or odd under the charge conjugation operator. We develop a transport equation that illustrates these ideas and discuss its experimental and theoretical consequences.

cond-mat↗

arXiv1996

Sum rule for the optical Hall angle

We consider the optical Hall conductivity of a general electronic medium and prove that the optical Hall angle obeys a new sum rule. This sum rule governs the response of an electronic fluid to a Lorentz electric field and can thought of as the transverse counterpart to the f-sum rule in optical conductivity. The physical meaning of this sum rule is discussed, giving a number of examples of its application to a variety of of electronic media.

cond-mat↗

arXiv1996

Thermal currents in highly correlated systems

Conventional approaches to thermal conductivity in itinerant systems neglect the contribution to thermal current due to interactions. We derive this contribution to the thermal current and show how it produces important corrections to the thermal conductivity in anisotropic superconductors. We discuss the possible relevance of these corrections for the interpretation of the thermal conductivity of anisotropic superconductors.

cond-mat↗

arXiv1996

Phenomenological Transport Equation for the Cuprate Metals

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