T. Lühmann

Hall-effect evolution across a heavy-fermion quantum critical point

A quantum critical point (QCP) develops in a material at absolute zero when a new form of order smoothly emerges in its ground state. QCPs are of great current interest because of their singular ability to influence the finite temperature properties of materials. Recently, heavy-fermion metals have played a key role in the study of antiferromagnetic QCPs. To accommodate the heavy electrons, the Fermi surface of the heavy-fermion paramagnet is larger than that of an antiferromagnet. An important unsolved question is whether the Fermi surface transformation at the QCP develops gradually, as expected if the magnetism is of spin-density-wave (SDW) type, or suddenly, as expected if the heavy electrons are abruptly localized by magnetism. Here we report measurements of the low-temperature Hall coefficient (RH)—a measure of the Fermi surface volume—in the heavy-fermion metal YbRh2Si2 upon field-tuning it from an antiferromagnetic to a paramagnetic state. RH undergoes an increasingly rapid change near the QCP as the temperature is lowered, extrapolating to a sudden jump in the zero temperature limit. We interpret these results in terms of a collapse of the large Fermi surface and of the heavy-fermion state itself precisely at the QCP.

Ferromagnetic Quantum Critical Point in the Heavy-Fermion Metal YbNi4(P1−xAsx)2

Arsenic Makes a Difference A quantum critical point (QCP) occurs when quantum fluctuations, which do not go away even at absolute zero, cause a gradual (so-called second order) phase change. QCPs have been observed in ferromagnets, but for ferromagnetic metals, the evidence is less clear-cut and it is thought that, as the temperature is lowered, another order—such as superconductivity—will prevent the formation of a QCP. However, Steppke et al. (p. 933), using specific heat and magnetic susceptibility measurements, found strong evidence for a QCP in a quasi–one-dimensional heavy fermion material, YbNi4(P1−xAsx)2, near an Arsenic substitution level of about 10%. The results present a challenge to theories about quantum criticality in ferromagnets. Precision low-temperature measurements reveal a divergence associated with quantum criticality in a ferromagnetic metal. Unconventional superconductivity and other previously unknown phases of matter exist in the vicinity of a quantum critical point (QCP): a continuous phase change of matter at absolute zero. Intensive theoretical and experimental investigations on itinerant systems have shown that metallic ferromagnets tend to develop via either a first-order phase transition or through the formation of intermediate superconducting or inhomogeneous magnetic phases. Here, through precision low-temperature measurements, we show that the Grüneisen ratio of the heavy fermion metallic ferromagnet YbNi4(P0.92As0.08)2 diverges upon cooling to T = 0, indicating a ferromagnetic QCP. Our observation that this kind of instability, which is forbidden in d-electron metals, occurs in a heavy fermion system will have a large impact on the studies of quantum critical materials.

Experimental evidence for a generalized FFLO state in clean type-II superconductors with short coherence length and enhanced Pauli susceptibility

Abstract We report an investigation of the magnetic and dilatometric properties of single crystals of the superconductors UPd 2 Al 3 and CeRu 2 , both compounds exhibiting enhanced spin susceptibilities. Our results suggest for both systems a first-order transition between weak and strong pinning at T T c , somewhat below H c2 ( T ). We argue that these observations are compatible with a staggered order parameter due to the formation of a “generalized Fulde-Ferrell-Larkin-Ovchinnikov state”.

A compensated heat-pulse calorimeter for low temperatures

We describe a technique for measuring heat capacities (Cmin≈1 μJ/K at 0.1 K) of small solid samples at low temperatures (0.03 K<T<6 K) and in high magnetic fields (B<12 T). In this compensated heat-pulse technique the thermal losses are compensated through a background heating. A detailed analysis of the heat flow takes the heat input and losses into account. Test measurements on tin and YbRh2(Si0.95Ge0.05)2 showed that the heat capacity can be determined with high precision in a fast and accurate way. This technique provides a versatile calorimeter for a wide range of heat capacities which achieves its main performance if several sample platforms are mounted and one sample is measured while the other may cool down.

Unconventional Normal-State Properties and Superconductivity in Heavy-Fermion Metals

Abstract We discuss (i) UPd 2 Al 3 , for which local-moment antiferromagnetism ( T N = 14.3 K) coexists with heavy-fermion (HF) superconductivity below T c ≈ 2 K, (ii) the HF superconductor CeCu 2 Si 2 ( T c ≈ 0.6 K) which, for B > B c2 , shows pronounced “non-Fermi-liquid” (NFL) effects near a quantum-critical point at which the spin-density-wave “phase A” disappears, and (iii) the NFL CeNi 2 Ge 2 , a “clean-limit” HF superconductor below T c ≈ 0.1 K.

Doping effects on UPd2Al3

Abstract We present first results of a doping study on the U site on UPd2Al3. These results give further support for a tetravalent uranium configuration and an induced type of antiferromagnetic ordering. They confirm the importance of an unperturbed electronic configuration for both antiferromagnetic long-range ordering and heavy-fermion superconductivity. Implications for the interaction between both phenomena are discussed.

Anomalous pinning in superconductors with strong Pauli paramagnetism

Abstract The results of the magnetization, magnetostriction and AC-susceptibility experiments are presented for both the antiferromagnetic heavy-Fermion superconductor UPd 2 Al 3 and the valence fluctuation compound CeRu 2 . For ( H > 10 kOe). These results are discussed on the basis of a generalized Fulde-Ferrell-Larkin-Ovchinnikov superconducting state.

Bose-Einstein Condensation of Magnons in Cs2CuCl4

We report on results of specific heat measurements on single crystals of the frustrated quasi-2D spin-1/2 antiferromagnet Cs2CuCl4 (T(N)=0.595 K) in external magnetic fields B<12 T and for temperatures T>30 mK. Decreasing B from high fields leads to the closure of the field-induced gap in the magnon spectrum at a critical field Bc approximately = 8.51 T and a magnetic phase transition is clearly seen below Bc. In the vicinity of Bc, the phase transition boundary is well described by the power law Tc(B) proportional, variant (Bc-B)(1/phi), with the measured critical exponent phi approximately =1.5. These findings are interpreted as a Bose-Einstein condensation of magnons.

Anomalous pinning near Hc2 in the heavy fermion superconductor UPd2Al3 studied by magnetic measurements

Abstract Magnetic measurements were performed on single crystalline UPd2Al3 samples. Slightly below Hc2 an anomaly has been observed, which indicates a sudden change from weak to strong pinning of the flux vortices. We discuss possible explanations, e.g. the formation of spatially modulated superconducting (“Fulde-Ferrell”) state, and compare our results with similar ones in CeRu2.

Application of SQUIDs to low temperature and high magnetic field measurements-Ultra low noise torque magnetometry

Torque magnetometry is a key method to measure the magnetic anisotropy and quantum oscillations in metals. In order to resolve quantum oscillations in sub-millimeter sized samples, piezo-electric micro-cantilevers were introduced. In the case of strongly correlated metals with large Fermi surfaces and high cyclotron masses, magnetic torque resolving powers in excess of 104 are required at temperatures well below 1 K and magnetic fields beyond 10 T. Here, we present a new broadband read-out scheme for piezo-electric micro-cantilevers via Wheatstone-type resistance measurements in magnetic fields up to 15 T and temperatures down to 200 mK. By using a two-stage superconducting-quantum interference device as a null detector of a cold Wheatstone bridge, we were able to achieve a magnetic moment resolution of Δm = 4 × 10-15 J/T at maximal field and 700 mK, outperforming conventional magnetometers by at least one order of magnitude in this temperature and magnetic field range. Exemplary de Haas-van Alphen measurement of a newly grown delafossite, PdRhO2, was used to show the superior performance of our setup.