Optical Properties of Heavy Fermion Systems with SDW Order

The dynamical conductivity $\sigma (\omega)$, reflectivity $R(\omega)$, and tunneling density of states $N(\omega)$ of strongly correlated systems (like heavy fermions) with a spin-density wave (SDW) magnetic order are studied as a function of impurity scattering rate and temperature. The theory is generalized to include strong coupling effects in the SDW order. The results are discussed in the light of optical experiments on heavy-fermion SDW materials. With some modifications the proposed theory is applicable also to heavy fermions with localized antiferromagnetic (LAF) order.Comment: 9 pages, 10 figure

Magnetic state in URu2Si2, UPd2Al3 and UNi2Al3 probed by point contacts

The antiferromagnetic (AFM) state has been investigated in the three heavy-fermion compounds URu2Si2, UPd2Al3, and UNi2Al3 by measuring dV/dI(V) curves of point contacts at different temperatures (1.5-20 K) and magnetic fields (0-28 T). The zero-bias maximum in dV/dI(V) for URu2Si2 points to a partially gapped Fermi-surface related to the itinerant nature of the AFM state contrary to UPd2Al3 where analogous features have not been found. The AFM state in UNi2Al3 has more similarities with URu2Si2. For URu2Si2, the same critical field of about 40 T along the easy c axis is found for all features in dV/dI(V) corresponding to the Neel temperature, the gap in the electronic density of states, and presumably the ordered moments.Comment: 10 pages incl. 5 figures, LaTex 2

Confinement Effects in Antiferromagnets

Phase equilibrium in confined Ising antiferromagnets was studied as a function of the coupling (v) and a magnetic field (h) at the surfaces, in the presence of an external field H. The ground state properties were calculated exactly for symmetric boundary conditions and nearest-neighbor interactions, and a full zero-temperature phase diagram in the plane v-h was obtained for films with symmetry-preserving surface orientations. The ground-state analysis was extended to the H-T plane using a cluster-variation free energy. The study of the finite-T properties (as a function of v and h) reveals the close interdependence between the surface and finite-size effects and, together with the ground-state phase diagram, provides an integral picture of the confinement in anisotropic antiferromagnets with surfaces that preserve the symmetry of the order parameter.Comment: 10 pages, 8 figures, Accepted in Phys. Rev.

Surface induced disorder in body-centered cubic alloys

We present Monte Carlo simulations of surface induced disordering in a model of a binary alloy on a bcc lattice which undergoes a first order bulk transition from the ordered DO3 phase to the disordered A2 phase. The data are analyzed in terms of an effective interface Hamiltonian for a system with several order parameters in the framework of the linear renormalization approach due to Brezin, Halperin and Leibler. We show that the model provides a good description of the system in the vicinity of the interface. In particular, we recover the logarithmic divergence of the thickness of the disordered layer as the bulk transition is approached, we calculate the critical behavior of the maxima of the layer susceptibilities, and demonstrate that it is in reasonable agreement with the simulation data. Directly at the (110) surface, the theory predicts that all order parameters vanish continuously at the surface with a nonuniversal, but common critical exponent. However, we find different exponents for the order parameter of the DO3 phase and the order parameter of the B2 phase. Using the effective interface model, we derive the finite size scaling function for the surface order parameter and show that the theory accounts well for the finite size behavior of the DO3 ordering but not for that of B2 ordering. The situation is even more complicated in the neighborhood of the (100) surface, due to the presence of an ordering field which couples to the B2 order.Comment: To appear in Physical Review

Influence of a magnetic field on the antiferromagnetic order in UPt_3

A neutron diffraction experiment was performed to investigate the effect of a magnetic field on the antiferromagnetic order in the heavy fermion superconductor UPt_3. Our results show that a field in the basal plane of up to 3.2 Tesla, higher than H_c2(0), has no effect: it can neither select a domain nor rotate the moment. This has a direct impact on current theories for the superconducting phase diagram based on a coupling to the magnetic order.Comment: 7 pages, RevTeX, 3 postscript figures, submitted to Phys. Rev.

Superconductivity in Ce- and U-based "122" heavy-fermion compounds

This review discusses the heavy-fermion superconductivity in Ce- and U-based compounds crystallizing in the body-centered tetragonal ThCr2Si2 structure. Special attention will be paid to the theoretical background of these systems which are located close to a magnetic instability.Comment: 12 pages, 9 figures. Invited topical review (special issue on "Recent Developments in Superconductivity") Metadata and references update

Odd Frequency Pairing in the Kondo Lattice

We discuss the possibility that heavy fermion superconductors involve odd-frequency pairing of the kind first considered by Berezinskii. Using a toy model for odd frequency triplet pairing in the Kondo lattice we are able to examine key properties of this new type of paired state. To make progress treating the strong $n_f=1$ constraint in the Kondo lattice model we use the technical trick of a Majorana representation of the local moments, which permits variational treatments of the model without a Gutzwiller approximation. The simplest mean field theory involves the development of bound states between the local moments and conduction electrons, characterized by a spinor order parameter. We show that this state is a stable realization of odd frequency triplet superconductivity with surfaces of gapless excitations whose spin and charge coherence factors vanish linearly in the quasiparticle energy. A $T^3$ NMR relaxation rate coexists with a linear specific heat. We discuss possible extensions of our toy model to describe heavy fermion superconductivity.Comment: 67 page

Correlation functions near Modulated and Rough Surfaces

In a system with long-ranged correlations, the behavior of correlation functions is sensitive to the presence of a boundary. We show that surface deformations strongly modify this behavior as compared to a flat surface. The modified near surface correlations can be measured by scattering probes. To determine these correlations, we develop a perturbative calculation in the deformations in height from a flat surface. Detailed results are given for a regularly patterned surface, as well as for a self-affinely rough surface with roughness exponent $\zeta$. By combining this perturbative calculation in height deformations with the field-theoretic renormalization group approach, we also estimate the values of critical exponents governing the behavior of the decay of correlation functions near a self-affinely rough surface. We find that for the interacting theory, a large enough $\zeta$ can lead to novel surface critical behavior. We also provide scaling relations between roughness induced critical exponents for thermodynamic surface quantities.Comment: 31 pages, 2 figure

Magnetic Properties in Non-centrosymmetric Superconductors with and without Antiferromagnetic Order

The paramagnetic properties in non-centrosymmetric superconductors with and without antiferromagnetic (AFM) order are investigated with focus on the heavy Fermion superconductors, CePt_3Si, CeRhSi_3 and CeIrSi_3. First, we investigate the spin susceptibility in the linear response regime and elucidate the role of AFM order. The spin susceptibility at T=0 is independent of the pairing symmetry and increases in the AFM state. Second, the non-linear response to the magnetic field are investigated on the basis of an effective model for CePt_3Si which may be also applicable to CeRhSi_3 and CeIrSi_3. The role of antisymmetric spin-orbit coupling (ASOC), helical superconductivity, anisotropic Fermi surfaces and AFM order are examined in the dominantly s-, p- and d-wave states. We emphasize the qualitatively important role of the mixing of superconducting (SC) order parameters in the p-wave state which enhances the spin susceptibility and suppresses paramagnetic depairing effect in a significant way. Therefore, the dominantly p-wave superconductivity admixed with the s-wave order parameter is consistent with the paramagnetic properties of CePt_3Si at ambient pressure. We propose some experiments which can elucidate the novel pairing states in CePt_3Si as well as CeRhSi_3 and CeIrSi_3.Comment: To appear in J. Phys. Soc. Jpn. (2007) No.1

The underscreened Kondo effect: a two S=1 impurity model

The underscreened Kondo effect is studied within a model of two impurities S=1 interacting with the conduction band and via an interimpurity coupling $K\vec{S_1}.\vec{S_2}$. Using a mean-field treatment of the bosonized Hamiltonian, we show that there is no phase transition, but a continuous cross-over versus K from a non Kondo behaviour to an underscreened Kondo one. For a small antiferromagnetic coupling (K>0), a completely asymmetric situation is obtained with one s=${1/2}$ component strongly screened by the Kondo effect and the other one almost free to yield indirect magnetism, which shows finally a possible coexistence between a RKKY interaction and a local Kondo effect, as observed in Uranium compounds such as $UPt_3$.Comment: 27 pages, RevTeX, to be published in PR