Observation of an optical non-Fermi-liquid behavior in the heavy fermion state of YbRh2_{2}Si2_{2}

We report far-infrared optical properties of YbRh$_{2}$Si$_{2}$ for photon energies down to 2 meV and temperatures 0.4 -- 300 K. In the coherent heavy quasiparticle state, a linear dependence of the low-energy scattering rate on both temperature and photon energy was found. We relate this distinct dynamical behavior different from that of Fermi liquid materials to the non-Fermi liquid nature of YbRh$_{2}$Si$_{2}$ which is due to its close vicinity to an antiferromagnetic quantum critical point.Comment: 5 pages, 4 figures. submitte

Electron paramagnetic resonance of Yb3+ ions in a concentrated YbRh2Si2 compound with heavy fermions

The EPR signal from localized ytterbium ions was observed in an undoped YbRh2Si2 compound with heavy fermions in the temperature range from 1.5 to 25 K. The exponential contribution dominating the temperature dependence of EPR line width at temperatures above 15 K was shown to be caused by the random transitions from the ground to the first excited Stark sublevel of the Yb3+(4f13) ion with the activation energy Δ = 115 K. © 2003 MAIK "Nauka/Interperiodica"

Energy scales of Lu(1-x)Yb(x)Rh2Si2 by means of thermopower investigations

We present the thermopower S(T) and the resistivity rho(T) of Lu(1-x)Yb(x)Rh2Si2 in the temperature range 3 K < T < 300 K. S(T) is found to change from two minima for dilute systems (x < 0.5) to a single large minimum in pure YbRh2Si2. A similar behavior has also been found for the magnetic contribution to the resistivity rho_mag(T). The appearance of the low-T extrema in S(T) and rho_mag(T) is attributed to the lowering of the Kondo scale with decreasing x. The evolution of the characteristic energy scales for both the Kondo effect and the crystal electric field splitting are deduced. An extrapolation allows to estimate the Kondo temperature of YbRh2Si2 to 29 K.Comment: 15 pages, 4 figures, accepted in Phys. Rev.

Thermal evolution of cobalt deposits on Co3O4(111): atomically dispersed cobalt, two-dimensional CoO islands, and metallic Co nanoparticles

Cobalt oxide nanomaterials show high activity in several catalytic reactions thereby offering the potential to replace noble metals in some applications. We have developed a well-defined model system for partially reduced cobalt oxide materials aiming at a molecular level understanding of cobalt-oxide-based catalysis. Starting from a well-ordered Co3O4(111) film on Ir(100), we modified the surface by deposition of metallic cobalt. Growth, structure, and adsorption properties of the cobalt-modified surface were investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and infrared reflection absorption spectroscopy (IRAS) using CO as a probe molecule. The deposition of a submonolayer of cobalt at 300 K leads to the formation of atomically dispersed cobalt ions distorting the surface layer of the Co3O4 film. Upon annealing to 500 K the Co ions are incorporated into the surface layer forming ordered two-dimensional CoO islands on the Co3O4 grains. At 700 K, Co ions diffuse from the CoO islands into the bulk and the ordered Co3O4(111) surface is restored. Deposition of larger amounts of Co at 300 K leads to formation of metallic Co aggregates on the dispersed cobalt phase. The metallic particles sinter at 500 K and diffuse into the bulk at 700 K. Depending on the degree of bulk reduction, extended Co3O4 grains switch to the CoO(111) structure. All above structures show characteristic CO adsorption behavior and can therefore be identified by IR spectroscopy of adsorbed CO

Separation of energy scales in undoped YbRh2_2Si2_2 under hydrostatic pressure

The temperature ($T$)-magnetic field ($H$) phase diagram of YbRh$_2$Si$_2$ in the vicinity of its quantum critical point is investigated by low-$T$ magnetization measurements. Our analysis reveals that the energy scale $T^\star(H)$, previously related to the Kondo breakdown and terminating at 0.06 T for $T\to 0$, remains unchanged under pressure, whereas the antiferromagnetic critical field increases from 0.06 T ($p=0$) to 0.29 T ($p=1.28$ GPa), resulting in a crossing of $T_N(H)$ and $T^\star(H)$. Our results are very similar to those on Yb(Rh$_{1-x}$Co$_x$)$_2$Si$_2$, proving that the Co-induced disorder can not be the reason for the detachment of both scales under chemical pressure

Measurement of Angular Distributions and R= sigma_L/sigma_T in Diffractive Electroproduction of rho^0 Mesons

Production and decay angular distributions were extracted from measurements of exclusive electroproduction of the rho^0(770) meson over a range in the virtual photon negative four-momentum squared 0.5< Q^2 <4 GeV^2 and the photon-nucleon invariant mass range 3.8< W <6.5 GeV. The experiment was performed with the HERMES spectrometer, using a longitudinally polarized positron beam and a ^3He gas target internal to the HERA e^{+-} storage ring. The event sample combines rho^0 mesons produced incoherently off individual nucleons and coherently off the nucleus as a whole. The distributions in one production angle and two angles describing the rho^0 -> pi+ pi- decay yielded measurements of eight elements of the spin-density matrix, including one that had not been measured before. The results are consistent with the dominance of helicity-conserving amplitudes and natural parity exchange. The improved precision achieved at 47 GeV, reveals evidence for an energy dependence in the ratio R of the longitudinal to transverse cross sections at constant Q^2.Comment: 15 pages, 15 embedded figures, LaTeX for SVJour(epj) document class Revision: Fig. 15 corrected, recent data added to Figs. 10,12,14,15; minor changes to tex

Fermi-liquid instabilities at magnetic quantum phase transitions

This review discusses instabilities of the Fermi-liquid state of conduction electrons in metals with particular emphasis on magnetic quantum critical points. Both the existing theoretical concepts and experimental data on selected materials are presented; with the aim of assessing the validity of presently available theory. After briefly recalling the fundamentals of Fermi-liquid theory, the local Fermi-liquid state in quantum impurity models and their lattice versions is described. Next, the scaling concepts applicable to quantum phase transitions are presented. The Hertz-Millis-Moriya theory of quantum phase transitions is described in detail. The breakdown of the latter is analyzed in several examples. In the final part experimental data on heavy-fermion materials and transition-metal alloys are reviewed and confronted with existing theory.Comment: 62 pages, 29 figs, review article for Rev. Mod. Phys; (v2) discussion extended, refs added; (v3) shortened; final version as publishe

Measurement of the Proton Spin Structure Function g1p with a Pure Hydrogen Target

A measurement of the proton spin structure function g1p(x,Q^2) in deep-inelastic scattering is presented. The data were taken with the 27.6 GeV longitudinally polarised positron beam at HERA incident on a longitudinally polarised pure hydrogen gas target internal to the storage ring. The kinematic range is 0.021<x<0.85 and 0.8 GeV^2<Q^2<20 GeV^2. The integral Int_{0.021}^{0.85} g1p(x)dx evaluated at Q0^2 of 2.5 GeV^2 is 0.122+/-0.003(stat.)+/-0.010(syst.).Comment: 7 pages, 3 figures, 1 table, RevTeX late

Beam-Induced Nuclear Depolarisation in a Gaseous Polarised Hydrogen Target

Spin-polarised atomic hydrogen is used as a gaseous polarised proton target in high energy and nuclear physics experiments operating with internal beams in storage rings. When such beams are intense and bunched, this type of target can be depolarised by a resonant interaction with the transient magnetic field generated by the beam bunches. This effect has been studied with the HERA positron beam in the HERMES experiment at DESY. Resonances have been observed and a simple analytic model has been used to explain their shape and position. Operating conditions for the experiment have been found where there is no significant target depolarisation due to this effect.Comment: REVTEX, 6 pages, 5 figure