Interacting Dirac Materials

We investigate the extent to which the class of Dirac materials in two-dimensions provides general statements about the behavior of both fermionic and bosonic Dirac quasiparticles in the interacting regime. For both quasiparticle types, we find common features for the interaction induced renormalization of the conical Dirac spectrum. We perform the perturbative renormalization analysis and compute the self-energy for both quasiparticle types with different interactions and collate previous results from the literature whenever necessary. Guided by the systematic presentation of our results in Table~\ref{Summary}, we conclude that long-range interactions generically lead to an increase of the slope of the single-particle Dirac cone, whereas short-range interactions lead to a decrease. The quasiparticle statistics does not qualitatively impact the self-energy correction for long-range repulsion but does affect the behavior of short-range coupled systems, giving rise to different thermal power-law contributions. The possibility of a universal description of the Dirac materials based on these features is also mentioned.Comment: 19 pages and 12 Figures; Contains 6 Appendice

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.Comment: Latex file with two postscript figure

A Monte Carlo Event Generator for W Off-shell Pair Production including Higher Order Electromagnetic Radiative Corrections

We present the Monte Carlo event generator {\tt WOPPER} for pair production of $W$'s and their decays at high energy $e^+e^-$ colliders. {\tt WOPPER} includes the effects from finite $W$ width and focusses on the calculation of higher order electromagnetic corrections in the leading log approximation including soft photon exponentiation and explicit generation of exclusive hard photons.Comment: Contribution to the Second Workshop -- Munich, Annecy, Hamburg: $e^+e^-$ Collisions at 500~GeV: The Physics Potential, November 20, 1992, to April 3, 1993. LaTeX, 6 pages + 4 uuencoded EPS figures, IKDA 93/28, SI-93-

Charge ordering, stripes and phase separation in manganese perovskite oxides: an STM/STS study

A microscopic characterisation of the phase transitions associated with colossal magnetoresistance (CMR) in manganese perovskite oxides is a very important ingredient in the quest of understanding its underlying mechanism. Scanning tunneling microscopy (STM) is most suitable to investigate some of their reported hallmarks, including charge ordering, lattice distortions, and electronic phase separation. Here we investigate Bi1-XCaXMnO3 (BCMO) with x=0.76. At this composition, BCMO develops an insulating charge-ordered phase upon cooling, whose study as a function of temperature will allow identifying atomic scale characteristics of the metal-insulator phase transition (MIT). We observe distinct atomic scale phases at temperatures above and below the MIT, with very different electronic and structural characteristics. Combining STM micrographs and current-voltage tunneling characteristics, we find that charge ordering correlates both with the local conduction state (metallic or insulating) and the local structural order. Furthermore, STM shows coexistence of these phases as expected for a first order phase transition.Comment: Materials Science and Engineering C, (2005). In Pres

Anisotropy in magnetic and transport properties of Fe1-xCoxSb2

Anisotropic magnetic and electronic transport measurements were carried out on large single crystals of Fe1-xCoxSb2 (0<= x <=1). The semiconducting state of FeSb2 evolves into metallic and weakly ferromagnetic by substitution of Fe with Co for x<0.5. Further doping induces structural transformation from orthorhombic Pnnm structure of FeSb2 to monoclinic P21/c structure of CoSb2 where semiconducting and diamagnetic ground state is restored again. Large magnetoresistance and anisotropy in electronic transport were observed.Comment: 7 pages, 6 figure

Glassy relaxation without freezing in a random dipolar-coupled Ising magnet

We have measured the magnetic susceptibility, χ’+iχ’’, of the dilute dipolar-coupled Ising magnet LiHo_(0.045)Y_(0.955)F_4 over six decades of frequency from 0.02 Hz to 20 kHz. The system behaves as an ideal relaxational glass with Arrhenius behavior in temperature of the peak in χ’’. Scaling data from T=100 mK to T=300 mK by the peak in χ’’ shows an enhanced low-frequency response at high temperatures, in contrast to expectations for spin-glasses and random-field magnets

Ferromagnetism, glassiness, and metastability in a dilute dipolar-coupled magnet

We have measured the ac magnetic susceptibility of the model dilute dipolar-coupled Ising system LiHo_xY_(1−x)F_4. The x=0.46 material displays an ordinary ferromagnetic transition, while the x=0.045 and 0.167 samples are two very different magnetic glasses. Thermal relaxation times are more than five times longer for x=0.167 than for x=0.045. In addition, the more concentrated glass shows history dependence and metastability upon field cooling

A Comparison of the High-Frequency Magnetic Fluctuations in Insulating and Superconducting La2-xSrxCuO4

Inelastic neutron scattering performed at a spallation source is used to make absolute measurements of the dynamic susceptibility of insulating La2CuO4 and superconducting La2-xSrxCuO4 over the energy range 15<EN<350 meV. The effect of Sr doping on the magnetic excitations is to cause a large broadening in wavevector and a substantial change in the spectrum of the local spin fluctuations. Comparison of the two compositions reveals a new energy scale of 22 meV in La1.86Sr0.14CuO4.Comment: RevTex, 7 Pages, 4 postscript figure

Quantum Criticality in an Organic Magnet

Exchange interactions between $S=\frac{1}{2}$ sites in piperazinium hexachlorodicuprate produce a frustrated bilayer magnet with a singlet ground state. We have determined the field-temperature phase diagram by high field magnetization and neutron scattering experiments. There are two quantum critical points: $H_{c1}=7.5$ T separates a quantum paramagnet phase from a three dimensional, antiferromagnetically-ordered state while $H_{c2}=37$ T marks the onset of a fully polarized state. The ordered phase, which we describe as a magnon Bose-Einstein condensate (BEC), is embedded in a quantum critical regime with short range correlations. A low temperature anomaly in the BEC phase boundary indicates that additional low energy features of the material become important near $H_{c1}$.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett. Replaced original text with additional conten