Exotic Superconducting Properties in the Electron-Hole-Compensated Heavy-Fermion “Semimetal”URu2Si2

Abstract

We show that the charge and thermal transport measurements on ultraclean crystals of URu2Si2 reveal a number of unprecedented superconducting properties. The uniqueness is best highlighted by the peculiar field dependence of thermal conductivity including the first order transition at Hc2 with a reduction of entropy flow. This is a consequence of multi-band superconductivity with compensated electronic structure in the hidden order state of this system. We provide strong evidence for a new type of unconventional superconductivity with two distinct gaps having different nodal topology.The heavy-Fermion compound URu 2 Si 2 has mystified researchers since the superconducting state (T c = 1.5 K) is embedded within the "hidden order" phase (T h = 17.5 K) [1,2,3]. Although several exotic order parameters have been proposed for the hidden order phase [4], it is not identified yet. According to several experimental observations, most of the carriers disappear below T h resulting in a density one order of magnitude smaller than in other heavy-Fermion superconductors [5,6,7]. Superconductivity with such a low density is remarkablesince the superfluid density is very low in some way reminiscent of underdoped cuprates. Moreover, pressure studies reveal that the superconductivity coexists with the hidden order but is suppressed by antiferromagnetic ordering [8].In this Letter, using ultraclean single crystals, we report various anomalous superconducting properties in URu 2 Si 2 . We show that a peculiar electronic structure appearing below the hidden order transition provides an intriguing stage on which a new type of unconventional superconducting state appears.Single crystals of URu 2 Si 2 were grown by the Czochralski pulling method in a tetra-arc furnace. The welldefined superconducting transition was confirmed by the specific heat measurements. The thermal conductivity κ was measured using a standard four-wire steady state method along the a-axis (heat current q a). The contact resistance at low temperatures is less than 10 mΩ.We first discuss the electronic structure below T h . Figure 1 shows the temperature dependence of the resistivity ρ along the a-axis and Hall coefficient R H (solid circles) defined as R H ≡ dρxy dH at H → 0 T for H c in the tetragonal crystal structure. In zero field, ρ depends on T as ρ = ρ 0 + AT 2 below 6 K down to T c . The exceptionally low residual resistivity ρ 0 = 0.48 µΩ cm and large residual resistivity ratio RRR = 670 attest the highest crystal quality currently achievable. R H exhibits an eight-fold increase below T h and attains a T -independent value at low temperatures, associated with a strong reduction of the carrier density. Most remarkably, the magnetoresistance (MR) increases with decreasing temperature and becomes extremely large at the lowest temperatures. The inset of Fig.

High purity specimens of URu2Si2produced by a molten metal flux technique

“…Another important feature of the data shown in Figure 5 is that ρ(T ) increases strongly with increasing H . This behaviour has been observed previously and is related to the fact that URu 2 Si 2 is a compensated electron-hole semimetal, for which ρ(H )/ρ 0 is proportional to H 2 [7].…”

“…neutron scattering and μSR techniques yield ordered moments that are much too small to account for the entropy that is released at T 0 [4,5]. Considerable interest has also been paid to the superconducting state, which is clearly unconventional as evidenced by: (1) its occurrence deep inside the hidden order state [1][2][3], (2) the low carrier density from which it emerges [6,7], (3) the large upper critical field [8] and (4) measurements which suggest the presence of nodes in the superconducting energy gap [7,[9][10][11]. Furthermore, a variety of studies have shown that magnetism is found on the border of the hidden order state.…”

Strong correlation between anomalous quasiparticle scattering and unconventional superconductivity in the hidden-order phase of URu2Si2

“…1 The compound undergoes a second order phase transition at T 0 = 17.5 K, and this ordered state coexists with the unconventional superconductivity of chiral d-wave symmetry. 2 Although many theoretical models have been proposed for the ordered state, 3 the nature of the state is still not understood and is known as "hidden order" (HO). Applying pressure to URu 2 Si 2 induces a first order phase transition from the HO to a large moment antiferromagnetic (LAFM) state at P x = 0.5-0.9 GPa.…”

“…On the contrary, recent Fermi surface studies under high pressure across P x suggest the similarity of the Fermi surface topology between the HO and LAFM phases. 17,18 To gain further insights into the electrical transport in URu 2 Si 2 , we now analyze the data in the temperature regions from T l to 3.0 K using the expression ρ = ρ 0 + α 1 T + α 2 T 2 that has been used in the analysis of the anomalous electrical transport in the organic superconductors, the iron pnictide superconductors, and the high-T c cuprate superconductors. [19][20][21] The expression assumes two independent scattering rates: one is an isotropic rate in k space that gives the T 2 term in the resistivity and the other is anisotropic one that does the T -linear term.…”

Non-Fermi Liquid Regimes and Superconductivity in the Low Temperature Phase Diagrams of Strongly Correlated d- and f-Electron Materials

“…The electrical resistivity ρ(T ) has a negative slope between room temperature and 75 K, where it reaches a maximum and then decreases dramatically, which may be attributed to Kondo coherence. Well below the HO transition temperature, ρ(T ) has been described both by T 2 Fermi liquid behavior [92][93][94], and T n dependence, where n < 2 [95,96]. Given the presence of the two phase transitions, the electronic specific heat C(T ) is also somewhat complicated.…”