Normal-state electrical resistivity and superconducting magnetic penetration depth in Eu0.5K0.5Fe2As2

We report measurements of the temperature dependence of the electrical resistivity, \rho(T), and magnetic penetration depth, \lambda(T), for polycrystalline samples of Eu_0.5K_0.5Fe_2As_2 with Tc=31 K. $\rho(T)$ follows a linear temperature dependence above T_c and bends over to a weaker temperature dependence around 150 K. The magnetic penetration depth, determined by radio frequency technique displays an unusual minimum around 4 K which is associated with short-range ordering of localized Eu3+ moments.Comment: 7 pages 4 figure

Thermoelectric Behaviour Near Magnetic Quantum Critical Point

We use the coupled 2d-spin-3d-fermion model proposed by Rosch {\sl et. al.} (Phys. Rev. Lett. {\bf 79}, 159 (1997)) to study the thermoelectric behaviour of a heavy fermion compound when it is close to an antiferromagnetic quantum critical point. When the low energy spin fluctuations are quasi two dimensional, as has been observed in ${\rm YbRh}_2{\rm Si}_2$ and ${\rm CeCu}_{6-x}{\rm Au}_x$, with a typical 2d ordering wavevector and 3d Fermi surface, the ``hot'' regions on the Fermi surface have a finite area. Due to enhanced scattering with the nearly critical spin fluctuations, the electrons in the hot region are strongly renormalized. We argue that there is an intermediate energy scale where the qualitative aspects of the renormalized hot electrons are captured by a weak-coupling perturbative calculation. Our examination of the electron self energy shows that the entropy carried by the hot electrons is larger than usual. This accounts for the anomalous logarithmic temperature dependence of specific heat observed in these materials. We show that the same mechanism produces logarithmic temperature dependence in thermopower. This has been observed in ${\rm CeCu}_{6-x}{\rm Au}_x$. We expect to see the same behaviour from future experiments on ${\rm YbRh}_2{\rm Si}_2$.Comment: RevTex, two-column, 7 pages, 2 figure

Investigation of Single Boron Acceptors at the Cleaved Si:B (111) Surface

The cleaved and (2 x 1) reconstructed (111) surface of p-type Si is investigated by scanning tunneling microscopy (STM). Single B acceptors are identified due to their characteristic voltage-dependent contrast which is explained by a local energetic shift of the electronic density of states caused by the Coulomb potential of the negatively charged acceptor. In addition, detailed analysis of the STM images shows that apparently one orbital is missing at the B site at sample voltages of 0.4 - 0.6 V, corresponding to the absence of a localized dangling-bond state. Scanning tunneling spectroscopy confirms a strongly altered density of states at the B atom due to the different electronic structure of B compared to Si.Comment: 6 pages, 7 figure

Some remarks about pseudo gap behavior of nearly antiferromagnetic metals

In the antiferromagnetically ordered phase of a metal, gaps open on parts of the Fermi surface if the Fermi volume is sufficiently large. We discuss simple qualitative and heuristic arguments under what conditions precursor effects, i.e. pseudo gaps, are expected in the paramagnetic phase of a metal close to an antiferromagnetic quantum phase transition. At least for weak interactions, we do not expect the formation of pseudo gaps in a three dimensional material. According to our arguments, the upper critical dimension d_c for the formation of pseudo gaps is d_c=2. However, at the present stage we cannot rule out a higher upper critical dimension, 2 < d_c <= 3. We also discuss briefly the role of statistical interactions in pseudo gap phases.Comment: 6 pages, accepted in PRB, relevant references added, several small change

Magnetic-field- and temperature-dependent fermi surface of CeBiPt

The half-Heusler compounds CeBiPt and LaBiPt are semimetals with very low charge-carrier concentrations as evidenced by Shubnikov–de Haas (SdH) and Hall-effect measurements. Neutron-scattering results reveal a simple antiferromagnetic structure in CeBiPt below TN = 1.15 K. The band structure of CeBiPt sensitively depends on temperature, magnetic field and stoichiometry. Above a certain, sample-dependent, threshold field (B>25 T), the SdH signal disappears and the Hall coefficient reduces significantly. These effects are absent in the non-4f compound LaBiPt. Electronic-band-structure calculations can well explain the observed behaviour by a 4f-polarization-induced Fermi-surface modification

Magnetic-field- and temperature-dependent fermi surface of CeBiPt

The half-Heusler compounds CeBiPt and LaBiPt are semimetals with very low charge-carrier concentrations as evidenced by Shubnikov–de Haas (SdH) and Hall-effect measurements. Neutron-scattering results reveal a simple antiferromagnetic structure in CeBiPt below TN = 1.15 K. The band structure of CeBiPt sensitively depends on temperature, magnetic field and stoichiometry. Above a certain, sample-dependent, threshold field (B>25 T), the SdH signal disappears and the Hall coefficient reduces significantly. These effects are absent in the non-4f compound LaBiPt. Electronic-band-structure calculations can well explain the observed behaviour by a 4f-polarization-induced Fermi-surface modification

Transition from a strong-coupling fixed point to an intermediate-coupling fixed point in a single-channel SU(N) Kondo model: role of the filling and two-stage screening

We study an extended SU(N) single-impurity Kondo model in which the impurity spin is described by a combination of Abrikosov fermions and Schwinger bosons. Our aim is to describe both the quasiparticle-like excitations and the locally critical modes observed in various physical situations, including non-Fermi liquid behavior in heavy fermion systems in the vicinity of a quantum critical point. We identify the strong coupling fixed point of the model and study its stability within second order perturbation theory. Already in the single channel case and in contrast with either the pure bosonic or the pure fermionic case, the strong coupling fixed point is unstable against the conduction electron kinetic term as soon as the amount of Abrikosov fermions reaches a critical value. In the stability region, the partially screened, dressed impurity at site 0 repels the conduction electrons on adjacent sites. In the instability region, the impurity tends to attract $(N-1)$ conduction electrons to the neighboring sites, giving rise to a two-stage Kondo effect with additional screening.This result opens the route to the existence of an intermediate coupling fixed point, characterized by non-Fermi liquid behavior

Local fluctuations in quantum critical metals

We show that spatially local, yet low-energy, fluctuations can play an essential role in the physics of strongly correlated electron systems tuned to a quantum critical point. A detailed microscopic analysis of the Kondo lattice model is carried out within an extended dynamical mean-field approach. The correlation functions for the lattice model are calculated through a self-consistent Bose-Fermi Kondo problem, in which a local moment is coupled both to a fermionic bath and to a bosonic bath (a fluctuating magnetic field). A renormalization-group treatment of this impurity problem--perturbative in $\epsilon=1-\gamma$, where $\gamma$ is an exponent characterizing the spectrum of the bosonic bath--shows that competition between the two couplings can drive the local-moment fluctuations critical. As a result, two distinct types of quantum critical point emerge in the Kondo lattice, one being of the usual spin-density-wave type, the other ``locally critical.'' Near the locally critical point, the dynamical spin susceptibility exhibits $\omega/T$ scaling with a fractional exponent. While the spin-density-wave critical point is Gaussian, the locally critical point is an interacting fixed point at which long-wavelength and spatially local critical modes coexist. A Ginzburg-Landau description for the locally critical point is discussed. It is argued that these results are robust, that local criticality provides a natural description of the quantum critical behavior seen in a number of heavy-fermion metals, and that this picture may also be relevant to other strongly correlated metals.Comment: 20 pages, 12 figures; typos in figure 3 and in the main text corrected, version as publishe

Non-Fermi liquid behavior from two-dimensional antiferromagnetic fluctuations: a renormalization-group and large-N analysis

We analyze the Hertz-Moriya-Millis theory of an antiferromagnetic quantum critical point, in the marginal case of two dimensions (d=2,z=2). Up to next-to-leading order in the number of components (N) of the field, we find that logarithmic corrections do not lead to an enhancement of the Landau damping. This is in agreement with a renormalization-group analysis, for arbitrary N. Hence, the logarithmic effects are unable to account for the behavior reportedly observed in inelastic neutron scattering experiments on CeCu_{6-x}Au_x. We also examine the extended dynamical mean-field treatment (local approximation) of this theory, and find that only subdominant corrections to the Landau damping are obtained within this approximation, in contrast to recent claims.Comment: 15 pages, 8 figure

Conductance of Mesoscopic Systems with Magnetic Impurities

We investigate the combined effects of magnetic impurities and applied magnetic field on the interference contribution to the conductance of disordered metals. We show that in a metal with weak spin-orbit interaction, the polarization of impurity spins reduces the rate of electron phase relaxation, thus enhancing the weak localization correction to conductivity. Magnetic field also suppresses thermal fluctuations of magnetic impurities, leading to a recovery of the conductance fluctuations. This recovery occurs regardless the strength of the spin-orbit interaction. We calculate the magnitudes of the weak localization correction and of the mesoscopic conductance fluctuations at an arbitrary level of the spin polarization induced by a magnetic field. Our analytical results for the ``$h/e$'' Aharonov-Bohm conductance oscillations in metal rings can be used to extract spin and gyromagnetic factor of magnetic impurities from existing experimental data.Comment: 18 pages, 8 figure