T. Westerkamp

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

The tetragonal heavy-fermion (HF) metal YbRh2Si2 (Kondo temperature TK≈ 25 K) exhibits a magnetic field-induced quantum critical point related to the suppression of very weak antiferromagnetic (AF) ordering (TN = 70 mK) at a critical field of Bc = 0.06 T (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 B > Bc, 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* = 10 T, 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 50 T with that of the isoelectronic HF system YbIr2Si2, which, due to a larger unit-cell volume, has an enhanced TK of about 40 K.

Interplay between Kondo suppression and Lifshitz transitions in YbRh2Si2 at high magnetic fields.

We investigate the magnetic field dependent thermopower, thermal conductivity, resistivity, and Hall effect in the heavy fermion metal YbRh2Si2. In contrast to reports on thermodynamic measurements, we find in total three transitions at high fields, rather than a single one at 10 T. Using the Mott formula together with renormalized band calculations, we identify Lifshitz transitions as their origin. The predictions of the calculations show that all experimental results rely on an interplay of a smooth suppression of the Kondo effect and the spin splitting of the flat hybridized bands.

Ferromagnetic quantum criticality in the alloy CePd1-xRhx

CePd_1-xRh_x alloys exhibit a continuous evolution from ferromagnetism (T_C= 6.5 K) at x = 0 to a mixed valence (MV) state at x = 1. We have performed a detailed investigation on the suppression of the ferromagnetic (F) phase in this alloy using dc-(\chi_dc) and ac-susceptibility (\chi_ac), specific heat (C_m), resistivity (\rho) and thermal expansion (\beta) techniques. Our results show a continuous decrease of T_C (x) with negative curvature down to T_C = 3K at x*= 0.65, where a positive curvature takes over. Beyond x*, a cusp in cac is traced down to T_C* = 25 mK at x = 0.87, locating the critical concentration between x = 0.87 and 0.90. The quantum criticality of this region is recognized by the -log(T/T_0) dependence of C_m/T, which transforms into a T^-q (~0.5) one at x = 0.87. At high temperature, this system shows the onset of valence instability revealed by a deviation from Vegards law (at x_V~0.75) and increasing hybridization effects on high temperature \chi_dc and \rho. Coincidentally, a Fermi liquid contribution to the specific heat arises from the MV component, which becomes dominant at the CeRh limit. In contrast to antiferromagnetic systems, no C_m/T flattening is observed for x>x_cr rather the mentioned power law divergence, which coincides with a change of sign of \beta. The coexistence of F and MV components and the sudden changes in the T dependencies are discussed in the context of randomly distributed magnetic and Kondo couplings.

Single-crystal growth of YbRh2Si2 and YbIr2Si2

We report on the single-crystal growth of the heavy-fermion compounds YbRh2Si2 and YbIr2Si2 using a high-temperature indium-flux technique. The optimization of the initial composition and the temperature–time profile lead to large (up to 100 mg) and clean (ρ0 ≈ 0.5 µΩ cm) single crystals of YbRh2Si2. Low-temperature resistivity measurements revealed a sample-dependent temperature exponent below 10 K, which for the samples with highest quality deviates from a linear-in-T behaviour. Furthermore, we grew single crystals of the alloy series Yb(Rh1−x Ir x )2Si2 with 0 ≤ x ≤ 0.23 and report the structural details. For pure YbIr2Si2, we establish the formation of two crystallographic modifications, where the magnetic 4f electrons have different physical ground states.

Magnetization study of the energy scales in YbRh2Si2 under chemical pressure

We present a systematic study of the magnetization in YbRh2Si2 under slightly negative (6% Ir substitution) and positive (7% Co substitution) chemical pressure. We show how the critical field H0, associated with the high-field Lifshitz transitions, is shifted to lower (higher) values with Co (Ir) substitution. The critical field HN, which identifies the boundary line of the antiferromagnetic (AFM) phase TN(H) increases with positive pressure and it approaches zero with 6% Ir substitution. On the other side, the crossover field H*, associated with the energy scale T*(H) where a reconstruction of the Fermi surface has been observed, is not much influenced by the chemical substitution. Following the analysis proposed in Refs. 1–4 we have fitted the quantity with a crossover function to indentify H*. The T*(H) line follows an almost linear H-dependence at sufficiently high fields outside the AFM phase, but it deviates from linearity at T ≤ TN(0) and in Yb(Rh0.93Co0.07)2Si2 it changes slope clearly inside the AFM phase. Moreover, the full width at half maximum (FWHM) of the fit function depends linearly on temperature outside the phase, but remains constant inside, suggesting either that such an analysis is valid only for T ≥ TN(0) or that the Fermi surface changes continuously at T = 0 inside the AFM phase.

Break Up of Heavy Fermions at an Antiferromagnetic Instability

We present results of high-resolution, low-temperature measurements of the Hall coefficient, thermopower, and specific heat on stoichiometric YbRh 2 Si 2 . They support earlier conclusions of an electronic (Kondo-breakdown) quantum critical point concurring with a field induced antiferromagnetic one. We also discuss the detachment of the two instabilities under chemical pressure. Volume compression/expansion (via substituting Rh by Co/Ir) results in a stabilization/weakening of magnetic order. Moderate Ir substitution leads to a non-Fermi-liquid phase, in which the magnetic moments are neither ordered nor screened by the Kondo effect. The so-derived zero-temperature global phase diagram promises future studies to explore the nature of the Kondo breakdown quantum critical point without any interfering magnetism.

Magnetization measurements on YbRh2Si2 at very low temperatures

YbRh2Si2, a heavy fermion compound, is in the center of interest for its unconventional behavior around a quantum critical point (QCP) which can be tuned by a magnetic field. To study the system at the lowest possible temperatures, we have measured the dc magnetization in magnetic fields up to 60 mT and down to 1 mK using an rf SQUID magnetometer. Our fields were high enough to reach the QCP in the B ∥ (a, b) crystal orientation. Both field cooled (fc) and zero field cooled (zfc) data were taken. We found a sharp transition to a low magnetization ground state at 2.2 mK and differences between the fc – zfc traces below 11 mK. In this temperature range down to 2.2 mK an additional increase of the magnetization is seen, just before the final drop sets in. These results will be discussed with respect to the nature of the ground state in low magnetic fields and its relation to the QCP.

Magnetization Measurements on YbRh2Si2 at Very Low Temperatures

YbRh2Si2, a heavy fermion compound, is in the center of interest for its unconventional behavior around a quantum critical point (QCP) which can be tuned by a magnetic field. To study the system at the lowest possible temperatures, we have measured the dc magnetization in magnetic fields up to 60 mT and down to 1 mK using an rf SQUID magnetometer. Our fields were high enough to reach the QCP in the B ∥ (a, b) crystal orientation. Both field cooled (fc) and zero field cooled (zfc) data were taken. We found a sharp transition to a low magnetization ground state at 2.2 mK and differences between the fc – zfc traces below 11 mK. In this temperature range down to 2.2 mK an additional increase of the magnetization is seen, just before the final drop sets in. These results will be discussed with respect to the nature of the ground state in low magnetic fields and its relation to the QCP.