Strain enhancement of superconductivity in CePd₂Si₂under pressure

Abstract

We report resistivity and calorimetric measurements on two single crystals of CePd2Si2 pressurized up to 7.4 GPa. A weak uniaxial stress induced in the pressure cell demonstrates the sensitivity of the physics to anisotropy. Stress applied along the c-axis extends the whole phase diagram to higher pressures and enhances the superconducting phase emerging around the magnetic instability, with a 40% increase of the maximum superconducting temperature, Tc, and a doubled pressure range. Calorimetric measurements demonstrate the bulk nature of the superconductivity. PACS numbers:By varying an external control parameter, such as magnetic field, composition or pressure, many heavyfermion systems may be pushed through a quantum critical point (QCP), where their magnetic ordering temperature goes to zero. In the immediate vicinity of this point, transport and thermodynamic measurements show striking deviations from standard Fermi-liquid behavior [1,2,3,4,5,6,7]. In particular, the low temperature resistivity, ρ(T ), exhibits a T n behavior with 1<n≤1.5 over a wide temperature range. The nature of this non-Fermi-liquid (NFL) behavior remains an open question [8]. Is the spin-fluctuation description [1] appropriate, with itinerant magnetism developing below a characteristic temperature such as the Kondo temperature, T K ? Alternatively, this characteristic temperature may collapse at the QCP, leading to localized magnetism down to the lowest temperatures [2,8]. Particular attention has been paid to the case of stoichiometric compounds, whose weak disorder permits the observation of superconductivity around the magnetic instability. Due to the enhancement of low-lying magnetic excitations in this region, it is commonly believed that Cooper pairs are magnetically formed. In one of these systems, CePd 2 Si 2 , superconductivity was discovered in a window of about 1 GPa around the QCP at a critical pressure P c ≃ 2.8 GPa. Simultaneously, NFL behavior was found in resistivity measurements with a T 1.2−1.3 law over two decades in temperature [4,5,6,7]. As an exponent d/2 is predicted for a d-dimensional antiferromagnet by spin-fluctuation theory [9,10,11], it has been suggested that the magnetic excitation spectrum has an effective dimension close to 2. This assumption is supported by the quasi-linear pressure dependence of the Néel temperature, T N , predicted to be (P c − P ) 2/d , by the tetragonal symmetry (I4/mmm) and by the magnetic structure containing a frustrated moment in the center of the elementary cell [4].The results quoted above were obtained in hydrostatic conditions using a "liquid" pressure transmitting medium. Another investigation [7] was carried out in a Bridgman anvil cell, using a soft solid (steatite) as a pressure transmitting medium [12]. This showed a rather different phase diagram: around a higher critical pressure P c ≃ 3.6 GPa, a strikingly expanded superconducting region was found, lying from 2 to 7 GPa with a maximum of T c apparently disconnected from P c , casting doubt upon spin fluctuations as...

Electrical resistivity study of CeZn11: Magnetic field and pressure phase diagram up to 5 GPa

“…A similar influence of the CEF levels on the electrical resistivity can be observed in many other cerium based compounds. [29][30][31][32][33][34][35][36][37][38][39][40][41] The two anomalies we observe are expected to merge under pressure with the increase of the Kondo effect 42 and indeed, at a qualitative level, this is observed in Fig. 1(b) as the pressure is increased.…”

“…This value for CeZn 11 can be compared to Ce 3 Pt 4 In 13 ( d|J ex | dp ≈ 8.4 meV/GPa), which orders antiferromagnetically below 0.95 K. 38 It would be interesting to compare these values to other cerium compounds, especially members of the CeT 2 (Si,Ge) 2 (T = Cu,Pd,Au). [31][32][33][34][35] The knowledge of the pressure dependence of J ex will be used later to estimate the variation of the magnetic ordering temperature T O 1 (p). We note that fitting down to 5 K with 1 as a free parameter gave essentially the same estimate of the variation of the magnetic ordering temperature T O 1 (p), and values for 1 between 14 and 18 K. However, as already mentioned, the data are not well reproduced at low temperatures.…”

Anisotropy, disorder, and superconductivity in CeCu2Si2under high pressure

“…The enhanced T onset c at very high pressure is a clear indication of the sensitivity of the high pressure superconducting state to anisotropy, either via the current direction, or anisotropic pressure conditions. Cell #2 was designed to test the effect of anisotropic stress, in analogy to similar work on CePd 2 Si 2 [14]. The two samples were aligned at right angles with respect to the cell axis.…”

“…In addition, the isostructural compound CePd 2 Si 2 has been shown to be extremely sensitive to uniaxial stress under pressure. Dramatic variations in T c in CePd 2 Si 2 result from a change in crystalline orientation with respect to small non-hydrostatic components in a Bridgman anvil pressure cell with a quasi-hydrostatic steatite medium [14].…”

Calorimetric and transport investigations ofCePd2+xGe2−xWilhelm1, Jaccard2 2002Phys. Rev. B

“…Its Sicounterpart, the HF system CePd 2 Si 2 exhibits a similar magnetic structure with T N ≈ 10 K [27,28]. Several groups have confirmed the occurrence of a superconducting ground state if the magnetic order is suppressed by external pressure (p c = 2.8 GPa) [3,4,5,29,30,31,32]. If the change of interatomic distances is the main source of altering the exchange coupling between 4f and conduction electrons, J, then CePd 2 Ge 2 should reveal a pronounced variation of T N (p).…”