Y. Tokiwa

High-field phase diagram of the heavy-fermion metal YbRh2Si2

The tetragonal heavy-fermion (HF) metal YbRh2Si2 (Kondo temperature TK≈ 25u2009K) exhibits a magnetic field-induced quantum critical point related to the suppression of very weak antiferromagnetic (AF) ordering (TN = 70u2009mK) at a critical field of Bc = 0.06u2009T (B⊥ c). To understand the influence of magnetic fields on quantum criticality and the Kondo effect, we study the evolution of various thermodynamic and magnetic properties upon tuning the system by magnetic field. At Bu2009>u2009Bc, the AF component of the quantum critical fluctuations becomes suppressed, and FM fluctuations dominate. Their polarization with magnetic field gives rise to a large increase of the magnetization. At B* = 10u2009T, the Zeeman energy becomes comparable to kB TK, and a steplike decrease of the quasi-particle mass deduced from the specific-heat coefficient indicates the suppression of HF behaviour. The magnetization M(B) shows a pronounced decrease in slope at B* without any signature of metamagnetism. The field dependence of the linear magnetostriction coefficient suggests an increase of the Yb-valency with field, reaching 3+ at high fields. A negative hydrostatic pressure dependence of B* is found, similar to that of the Kondo temperature. We also compare the magnetization behaviour in pulsed fields up to 50u2009T with that of the isoelectronic HF system YbIr2Si2, which, due to a larger unit-cell volume, has an enhanced TK of about 40u2009K.

Ferromagnetic quantum critical fluctuations in YbRh2(Si0.95Ge0.05)2.

The bulk magnetic susceptibility chi(T,B) of YbRh(2)(Si(0.95)Ge(0.05))(2) has been investigated close to the field-induced quantum critical point at B(c) = 0.027 T. For B < or= 0.05 T a Curie-Weiss law with a negative Weiss temperature is observed at temperatures below 0.3 K. Outside this region, the susceptibility indicates ferromagnetic quantum critical fluctuations, chi(T) proportional, variantT-0.6 above 0.3 K. At low temperatures the Pauli susceptibility follows chi(0) proportional, variant(B-B(c))(-0.6) and scales with the coefficient of the T(2) term in the electrical resistivity. The Sommerfeld-Wilson ratio is highly enhanced and increases up to 30 close to the critical field.

Divergence of the Magnetic Grüneisen Ratio at the Field-Induced Quantum Critical Point in YbRh2Si2

The heavy-fermion metal YbRh2Si2 is studied by low-temperature magnetization M(T) and specific-heat C(T) measurements at magnetic fields close to the quantum critical point (H_{c}=0.06 T, H perpendicularc). Upon approaching the instability, dM/dT is more singular than C(T), leading to a divergence of the magnetic Grüneisen ratio Gamma_{mag}=-(dM/dT)/C. Within the Fermi-liquid regime, Gamma_{mag}=-G_{r}(H-H_{c};{fit}) with G_{r}=-0.30+/-0.01 and H_{c};{fit}=(0.065+/-0.005) T which is consistent with scaling behavior of the specific-heat coefficient in YbRh2(Si0.95Ge0.05)_{2}. The field dependence of dM/dT indicates an inflection point of the entropy as a function of magnetic field upon passing the line T;{ small star, filled}(H) previously observed in Hall and thermodynamic measurements.

Field-Induced Suppression of the Heavy-Fermion State in YbRh2Si2

We present calculations of the magnetic-field-induced changes of the heavy quasiparticles in YbRh2Si2 which are reflected in thermodynamic and transport properties. The quasiparticles are determined by means of the renormalized band method. The progressive de-renormalization of the quasiparticles in the magnetic field is accounted for using field-dependent quasiparticle parameters deduced from numerical renormalization group studies. Consequences for the interpretation of experimental data are discussed.

Magnetic phase transitions in the two-dimensional frustrated quantum antiferromagnet Cs2CuCl4

We report magnetization and specific heat measurements in the two-dimensional frustrated spin-

Quantum bicriticality in the heavy-fermion metamagnet YbAgGe.

1∕2

Quasiparticle Entropy in the High-Field Superconducting Phase of CeCoIn5

Heisenberg antiferromagnet

Low-temperature thermodynamic properties of the heavy-fermion compound YbAgGe close to the field-induced quantum critical point

{mathrm{Cs}}_{2}mathrm{Cu}{mathrm{Cl}}_{4}

Anomalous Reduction of the Lorenz Ratio at the Quantum Critical Point in YbAgGe

at temperatures down to

Quantum criticality near the upper critical field of Ce2PdIn8

0.05phantom{rule{0.3em}{0ex}}mathrm{K}