Jinhyuk Lim

Upper critical field of KFe2As2 under pressure: A test for the change in the superconducting gap structure

We report measurements of electrical resistivity under pressure to 5.8 GPa, magnetization to 6.7 GPa, and ac susceptibility to 7.1 GPa in KFe2As2. The previously reported change of slope in the pressure dependence of the superconducting transition temperature Tc(p) at a pressure p*~1.8 GPa is confirmed, and Tc(p) is found to be nearly constant above p* up to 7.1 GPa. The T-p phase diagram is very sensitive to the pressure conditions as a consequence of the anisotropic uniaxial pressure dependence of Tc. Across p*, a change in the behavior of the upper critical field is revealed through a scaling analysis of the slope of Hc2 with the effective mass as determined from the A coefficient of the T2 term of the temperature-dependent resistivity. We show that this scaling provides a quantitative test for the changes of the superconducting gap structure and suggests the development of a kz modulation of the superconducting gap above p* as a most likely explanation.

Different routes to pressure-induced volume collapse transitions in gadolinium and terbium metals

(Received 3 May 2013; revised manuscript received 16 November 2013; published 3 December 2013) The sudden decrease in molar volume exhibited by most lanthanides under high pressure is often attributed to changes in the degree of localization of their 4f electrons. We give evidence, based on electrical resistivity measurements of dilute Y(Gd) and Y(Tb) alloys to 120 GPa, that the volume collapse transitions in Gd and Tb metals have different origins, despite their being neighbors in the periodic table. Remarkably, the change under pressure in the magnetic state of isolated Pr or Tb impurity ions in the nonmagnetic Y host appears to closely mirror corresponding changes in pure Pr or Tb metals. The collapse in Tb appears to be driven by an enhanced negative exchange interaction between 4f and conduction electrons under pressure (Kondo resonance) which, in the case of Y(Tb), dramatically alters the superconducting properties of the Y host, much like previously found for Y(Pr). In Gd, our resistivity measurements suggest that a Kondo resonance is not the main driver for its volume collapse. X-ray absorption and emission spectroscopies clearly show that 4f local moments remain largely intact across both volume collapse transitions ruling out 4f band formation (delocalization) and valence transition models as possible drivers. The results highlight the richness of behavior behind the volume collapse transition in lanthanides and demonstrate the stability of the 4f level against band formation to extreme pressure.

Magnetic ordering at anomalously high temperatures in Dy at extreme pressures

In an attempt to destabilize the magnetic state of the heavy lanthanide Dy, extreme pressures were applied in an electrical resistivity measurement to 157 GPa over the temperature range 1.3-295 K. The magnetic ordering temperature To and spin-disorder resistance Rsd of Dy, as well as the superconducting pair-breaking effectT c in Y(1 at.% Dy), are found to track each other in a highly nonmonotonic fashion as a function of pressure. Above 73 GPa, the critical pressure for a 6% volume collapse in Dy, all three quantities increase sharply (dT o/dP � 5. 3K /GPa), To appearing to rise above ambient temperature for P> 107 GPa. In contrast, To and �T c for Gd and Y(0.5 at.% Gd), respectively, show no such sharp increase with pressure (dTo/dP � 0.73 K/GPa). Taken together, these results suggest that extreme pressure transports Dy into an unconventional magnetic state with an anomalously high magnetic ordering temperature.

Electronic and structural ground state of heavy alkali metals at high pressure

(Received 21 December 2014; published 17 February 2015)Alkali metals display unexpected properties at high pressure, including emergence of low-symmetry crystalstructures, which appear to occur due to enhanced electronic correlations among the otherwise nearly freeconduction electrons. We investigate the high-pressure electronic and structural ground state of K, Rb, and Csusing x-ray absorption spectroscopy and x-ray diffraction measurements together with

Suppression of dense Kondo state in CeB6 under pressure

To investigate whether the dense Kondo compound CeB6 might evolve into a topological insulator under sufficient pressure, four-point electrical resistivity measurements have been carried out over the temperature range 1.3 K–295 K in a diamond anvil cell to 122 GPa. The temperature Tmax of the resistivity maximum initially increases slowly with pressure but disappears between 12 and 20 GPa. The marked changes observed under pressure suggest that a valence and/or structural transition may have occurred. Synchrotron x-ray diffraction measurements, however, fail to detect any change in crystal structure to 85 GPa. Although a transition into an insulating phase is not observed, this dense Kondo system is completely suppressed at 43 GPa, leaving behind what appears to be a conventional Fermi liquid metal.

Hydrostatic high-pressure studies to 25 GPa on the model superconducting pnictide LaRu2P2

Prior to the discovery of the Fe-pnictides in 2008, the ruthenium phosphide LaRu2P2 possessed the highest value of the superconducting transition temperature, Tc ≈ 4 K, in the entire pnictide family. Recently, there has been renewed interest in this compound in an effort to better understand why the Fe-pnictides have much higher values of Tc. In related phosphides superconductivity appears to only be present if the separation between the phosphor ions dp−p in neighboring Ru2P2 planes is greater than the critical value 2.8 A, too great for a P-P covalent bond to be formed. For example, in superconducting LaRu2P2, the value of dp−p is 3.0 A. To test these ideas directly, we have carried out hydrostatic high-pressure studies on single-crystalline LaRu2P2 in a diamond-anvil cell using He pressure medium to pressures as high as 25 GPa and temperatures as low as 1.5 K. We find that Tc initially increases under pressure, but suddenly disappears above 2.1 GPa. Since dp−p decreases under pressure, the sudden disappearance of superconductivity is likely due to the formation of a covalent P-P bond between adjacent Ru2P2 planes and a possible structural phase transition.

Anomalous pressure dependence of magnetic ordering temperature in Tb revealed by resistivity measurements to 141 GPa: Comparison with Gd and Dy

In previous studies, the pressure dependence of the magnetic ordering temperature To of Dy was found to exhibit a sharp increase above its volume collapse pressure of 73 GPa, appearing to reach temperatures well above ambient at 157 GPa. In a search for a second such lanthanide, electrical resistivity measurements were carried out on neighboring Tb to 141 GPa over the temperature range 3.8–295 K. Below Tb’s volume collapse pressure of 53 GPa, the pressure dependence To(P ) mirrors that of both Dy and Gd. However, at higher pressures To(P ) for Tb becomes highly anomalous. This result, together with the very strong suppression of superconductivity by dilute Tb ions in Y, suggests that extreme pressure transports Tb into an unconventional magnetic state with an anomalously high magnetic ordering temperature.