Unusual weak magnetic exchange in two different structure types: YbPt2_2Sn and YbPt2_2In

We present the structural, magnetic, thermodynamic, and transport properties of the two new compounds YbPt$_2$Sn and YbPt$_2$In. X-ray powder diffraction shows that they crystallize in different structure types, the hexagonal ZrPt$_2$Al and the cubic Heusler type, respectively. Despite quite different lattice types, both compounds present very similar magnetic properties: a stable trivalent Yb$^{3+}$, no evidence for a sizeable Kondo interaction, and very weak exchange interactions with a strength below 1K as deduced from specific heat $C(T)$. Broad anomalies in $C(T)$ suggest short range magnetic ordering at about 250mK and 180mK for YbPt$_2$Sn and YbPt$_2$In, respectively. The weak exchange and the low ordering temperature result in a large magnetocaloric effect as deduced from the magnetic field dependence of $C(T)$, making these compounds interesting candidates for magnetic cooling. In addition we found in YbPt$_2$In evidences for a charge density wave transition at about 290K. The occurrence of such transitions within several RET$_2$X compound series (RE = rare earth, T = noble metal, X = In, Sn) is analyzed.Comment: 16 pages, 7 figure

Emergence of superconductivity in the canonical heavy-electron metal YbRh2Si2

We report magnetic and calorimetric measurements down to T = 1 mK on the canonical heavy-electron metal YbRh2Si2. The data reveal the development of nuclear antiferromagnetic order slightly above 2 mK. The latter weakens the primary electronic antiferromagnetism, thereby paving the way for heavy-electron superconductivity below Tc = 2 mK. Our results demonstrate that superconductivity driven by quantum criticality is a general phenomenon.Comment: 39 pages including Supplementary Materials. Version before copy-edited by the journa

Bose glass and Mott glass of quasiparticles in a doped quantum magnet

The low-temperature states of bosonic fluids exhibit fundamental quantum effects at the macroscopic scale: the best-known examples are Bose-Einstein condensation (BEC) and superfluidity, which have been tested experimentally in a variety of different systems. When bosons are interacting, disorder can destroy condensation leading to a so-called Bose glass. This phase has been very elusive to experiments due to the absence of any broken symmetry and of a finite energy gap in the spectrum. Here we report the observation of a Bose glass of field-induced magnetic quasiparticles in a doped quantum magnet (Br-doped dichloro-tetrakis-thiourea-Nickel, DTN). The physics of DTN in a magnetic field is equivalent to that of a lattice gas of bosons in the grand-canonical ensemble; Br-doping introduces disorder in the hoppings and interaction strengths, leading to localization of the bosons into a Bose glass down to zero field, where it acquires the nature of an incompressible Mott glass. The transition from the Bose glass (corresponding to a gapless spin liquid) to the BEC (corresponding to a magnetically ordered phase) is marked by a novel, universal exponent governing the scaling on the critical temperature with the applied field, in excellent agreement with theoretical predictions. Our study represents the first, quantitative account of the universal features of disordered bosons in the grand-canonical ensemble.Comment: 13+6 pages, 5+6 figures; v2: Fig. 5 update

Microstructuring YbRh2Si2 for resistance and noise measurements down to ultra-low temperatures

We acknowledge funding by the German Research Foundation (DFG) via the TRR 288 (422213477, project A03, A10 and B02) and projects KR3831/4-1 and BR 4110/1-1. This work was supported by the EU H2020 European Microkelvin Platform EMP, Grant No. 824109.The discovery of superconductivity in the quantum critical Kondo-lattice system YbRh2Si2 at an extremely low temperature of 2 mK has inspired efforts to perform high-resolution electrical resistivity measurements down to this temperature range in highly conductive materials. Here we show that control over the sample geometry by microstructuring using focused-ion-beam techniques allows to reach ultra-low temperatures and increase signal-to-noise ratios (SNRs) tenfold, without adverse effects to sample quality. In five experiments we show four-terminal sensing resistance and magnetoresistance measurements which exhibit sharp phase transitions at the Néel temperature, and Shubnikov–de-Haas (SdH) oscillations between 13 T and 18 T where we identified a new SdH frequency of 0.39 kT. The increased SNR allowed resistance fluctuation (noise) spectroscopy that would not be possible for bulk crystals, and confirmed intrinsic 1/f -type fluctuations. Under controlled strain, two thin microstructured samples exhibited a large increase of TN from 67 mK up to 188 mK while still showing clear signatures of the phase transition and SdH oscillations. Superconducting quantum interference device-based thermal noise spectroscopy measurements in a nuclear demagnetization refrigerator down to 0.95 mK, show a sharp superconducting transition at Tc=1.2 mK. These experiments demonstrate microstructuring as a powerful tool to investigate the resistance and the noise spectrum of highly conductive correlated metals over wide temperature ranges.Publisher PDFPeer reviewe

Low temperature thermodynamic properties near the field-induced quantum critical point in DTN

We present a comprehensive experimental and theoretical investigation of the thermodynamic properties: specific heat, magnetization and thermal expansion in the vicinity of the field-induced quantum critical point (QCP) around the lower critical field $H_{c1} \approx 2$\,T in DTN . A $T^{3/2}$ behavior in the specific heat and magnetization is observed at very low temperatures at $H=H_{c1}$ that is consistent with the universality class of Bose-Einstein condensation of magnons. The temperature dependence of the thermal expansion coefficient at $H_{c1}$ shows minor deviations from the expected $T^{1/2}$ behavior. Our experimental study is complemented by analytical calculations and Quantum Monte Carlo simulations, which reproduce nicely the measured quantities. We analyze the thermal and the magnetic Gr\"{u}neisen parameters that are ideal quantities to identify QCPs. Both parameters diverge at $H_{c1}$ with the expected $T^{-1}$ power law. By using the Ehrenfest relations at the second order phase transition, we are able to estimate the pressure dependencies of the characteristic temperature and field scales.Comment: 11 paged, 10 figures, submitted to PR

Strong peak in Tc of Sr2RuO4 under uniaxial pressure

Sr2RuO4 is an unconventional superconductor that has attracted widespread study because of its high purity and the possibility that its superconducting order parameter has odd parity. We study the dependence of its superconductivity on anisotropic strain. Applying uniaxial pressures of up to ~1 gigapascals along a 〈100〉 direction (a axis) of the crystal lattice results in the transition temperature (Tc) increasing from 1.5 kelvin in the unstrained material to 3.4 kelvin at compression by ≈0.6%, and then falling steeply. Calculations give evidence that the observed maximum Tc occurs at or near a Lifshitz transition when the Fermi level passes through a Van Hove singularity, and open the possibility that the highly strained, Tc = 3.4 K Sr2RuO4 has an even-parity, rather than an odd-parity, order parameter.PostprintPeer reviewe

Field-induced double dome and Bose-Einstein condensation in the crossing quantum spin chain system AgVOAsO4

We present inelastic neutron scattering data on the quantum paramagnet AgVOAsO4 that establish the system is a S=1/2 alternating spin chain compound and provide a direct measurement of the spin gap. We also present experimental evidence for two different types of field-induced magnetic order between μ0Hc1= 8.4 T and μ0Hc2=48.9 T, which may be related to Bose-Einstein condensation (BEC) of triplons. Thermodynamic measurements in magnetic fields up to 60 T and temperatures down to 0.1 K reveal a H−T phase diagram consisting of a dome encapsulating two ordered phases with maximum ordering temperatures of 3.8 K and 5.3 K respectively. This complex phase diagram is not expected for a single-Q BEC system and therefore establishes AgVOAsO4 as a promising multi-Q BEC candidate capable of hosting exotic vortex phases

Single-ion Kondo Scaling of the Coherent Fermi Liquid Regime in Ce1-xLaxNi2Ge2

Thermodynamic and transport properties of the La-diluted Kondo lattice CeNi2Ge2 were studied in a wide temperature range. The Ce-rich alloys Ce1-xLaxNi2Ge2 were found to exhibit distinct features of the coherent heavy Fermi liquid. At intermediate compositions (0.7 <= x <= 0.9) non-Fermi liquid properties have been observed, followed by the local Fermi liquid behavior in the dilute limit. The 4f-electron contribution to the specific heat was found to follow the predictions of the Kondo impurity model both in the local as well as coherent regimes, with the characteristic Kondo temperature decreasing rapidly from about 30 K for the parent compound CeNi2Ge2 to about 1K in the most dilute samples. The specific heat does not show any evidence for the emergence of a new characteristic energy scale related to the formation of the coherent Kondo lattice.Comment: to appear in Physical Review Letter

Rigid platform for applying large tunable strains to mechanically delicate samples

The authors acknowledge the financial support from the Max Planck Society. J.P. acknowledges the financial support from the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (Grant No. 2016K1A4A4A01922028). Work in Japan was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Quantum Liquid Crystals” (Grant No. JP19H05824) from the Japan Society for the Promotion of Science.Response to uniaxial stress has become a major probe of electronic materials. Tunable uniaxial stress may be applied using piezoelectric actuators, and so far two methods have been developed to couple samples to actuators. In one, actuators apply force along the length of a free, beam-like sample, allowing very large strains to be achieved. In the other, samples are affixed directly to piezoelectric actuators, allowing the study of mechanically delicate materials. Here, we describe an approach that merges the two: thin samples are affixed to a substrate, which is then pressurized uniaxially using piezoelectric actuators. Using this approach, we demonstrate the application of large elastic strains to mechanically delicate samples: the van der Waals-bonded material FeSe and a sample of CeAuSb2 that was shaped with a focused ion beam.Publisher PDFPeer reviewe