RESUMO
Several quantum paramagnets exhibit magnetic-field-induced quantum phase transitions to an antiferromagnetic state that exists for H c1 ≤ H ≤ H c2. For some of these compounds, there is a significant asymmetry between the low- and high-field transitions. We present specific heat and thermal conductivity measurements in NiCl2-4SC(NH2)2, together with calculations which show that the asymmetry is caused by a strong mass renormalization due to quantum fluctuations for H ≤ H c1 that are absent for H ≥ H c2. We argue that the enigmatic lack of asymmetry in thermal conductivity is due to a concomitant renormalization of the impurity scattering.
RESUMO
Polarizability of noninteracting 2D Dirac electrons has a 1/square root(qv-omega) singularity at the boundary of electron-hole excitations. The screening of this singularity by long-range electron-electron interactions is usually treated within the random phase approximation. The latter is exact only in the limit of N-->infinity, where N is the "color" degeneracy. We find that the ladder-type vertex corrections become crucial close to the threshold as the ratio of the nth order ladder term to the same order random phase approximation contribution is ln(n)|qv-omega|/N(n). We perform an analytical summation of the infinite series of ladder diagrams which describe the excitonic effect. Beyond the threshold, qv>omega, the real part of the polarization operator is found to be positive leading to the appearance of a strong and narrow plasmon resonance.
