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With ${T}_{c}\ensuremath{\sim}9.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {\mathrm{Be}}_{22}\mathrm{Re}$ exhibits one of the highest critical temperatures among Be-rich compounds. We have carried out a series of high-pressure electrical resistivity measurements on this compound to 30 GPa. The data show that the critical temperature ${T}_{c}$ is suppressed gradually at a rate of $d{T}_{c}/dP=\ensuremath{-}0.05\phantom{\rule{0.16em}{0ex}}\mathrm{K}/\mathrm{GPa}$. Using density functional theory (DFT) calculations of the electronic and phonon density of states (DOS) and the measured critical temperature, we estimate that the rapid increase in lattice stiffening in ${\mathrm{Be}}_{22}\mathrm{Re}$ overwhelms a moderate increase in the electron-ion interaction with pressure, resulting in the decrease in ${T}_{c}$. High-pressure x-ray diffraction measurements show that the ambient pressure crystal structure of ${\mathrm{Be}}_{22}\mathrm{Re}$ persists to at least 154 GPa.

UBe13 was the second discovered heavy fermion superconductor, and numerous pieces of evidence exist that imply that it is an unconventional (non-BCS, non-s wave pairing symmetry) superconductor. Exhibiting even more signs of unconventional superconductivity, Th-doped UBe13 is perhaps the most puzzling of any of the unconventional superconductors. This review considers both the parent, undoped compound as well as the more interesting Th-doped UBe13. After summarizing the rather thorough characterization, which because of the interest in these compounds has continued from their discovery in 1983 and 1984 to date, these properties are compared with a recent template for determining whether a superconductor is unconventional. Finally, further experiments are suggested., accepted review to appear in Journal of Low Temperature Physics

Two-dimensional triangular-lattice materials with spin-1/2 are perfect platforms for investigating quantum frustrated physics with spin fluctuations. Here we report the structure, magnetization, heat capacity, and inelastic neutron scattering (INS) results on cesium ytterbium diselenide, ${\mathrm{CsYbSe}}_{2}$. There is no long-range magnetic order down to 0.4 K at zero field. The temperature-dependent magnetization, $M(T)$, reveals an easy-plane magnetic anisotropy. A maximum is found in $M(T)$ around $T\ensuremath{\approx}1.5$ K when magnetic field $H$ is applied in the $ab$ plane, indicating the short-range interaction. The low-temperature isothermal magnetization $M(H)$ shows a one-third plateau of the estimated saturation moment, which is characteristic of a two-dimensional (2D) frustrated triangular lattice. Heat capacity shows field-induced long-range magnetic order for both $H||c$ and $H||ab$ directions. The broad peak in heat capacity and highly damped INS magnetic excitation at $T=2$ K suggests strong spin fluctuations. The dispersive in-plane INS, centered at the (1/3 1/3 0) point, and the absence of dispersion along the $c$ direction suggest ${120}^{\ensuremath{\circ}}$ noncollinear 2D-like spin correlations. All these results indicate that the two-dimensional frustrated material ${\mathrm{CsYbSe}}_{2}$ can be in proximity to the triangular-lattice quantum spin liquid. We propose an experimental low-temperature $H\text{\ensuremath{-}}T$ phase diagram for ${\mathrm{CsYbSe}}_{2}$.

Predicting the critical temperature ${T}_{c}$ of new superconductors is a notoriously difficult task, even for electron-phonon paired superconductors, for which the theory is relatively well understood. Early attempts to obtain a simple ${T}_{c}$ formula consistent with strong-coupling theory, by McMillan and by Allen and Dynes, led to closed-form approximate relations between ${T}_{c}$ and various measures of the phonon spectrum and the electron-phonon interaction appearing in Eliashberg theory. Here we propose that these approaches can be improved with the use of machine-learning algorithms. As an initial test, we train a model for identifying low-dimensional descriptors using the ${T}_{c}l10$ K dataset by Allen and Dynes, and show that a simple analytical expression thus obtained improves upon the Allen-Dynes fit. Furthermore, the prediction for the recently discovered high-${T}_{c}$ material ${\mathrm{H}}_{3}\mathrm{S}$ at high pressure is quite reasonable. Interestingly, ${T}_{c}$'s for more recently discovered superconducting systems with a more two-dimensional electron-phonon coupling, which do not follow Allen and Dynes's expression, also do not follow our analytic expression. Thus, this machine-learning approach appears to be a powerful method for highlighting the need for a new descriptor beyond those used by Allen and Dynes to describe their set of isotropic electron-phonon coupled superconductors. We argue that this machine-learning method, and its implied need for a descriptor characterizing Fermi-surface properties, represents a promising approach to superconductor materials discovery which may eventually replace the serendipitous discovery paradigm begun by Kamerlingh Onnes.

Understanding the coexistence, competition and/or cooperation between superconductivity and charge density waves (CDWs) in the transition metal dichalcogenides (TMDs) is an elusive goal which, when realized, promises to reveal fundamental information on this important class of materials. Here, we use four-terminal current-voltage measurements to study the Van der Waals interface between freshly exfoliated flakes of the high-$T_{c}$ superconductor, Bi-2212, and the CDW-dominated TMD layered material, 1T-TaS$_{2}$. For highly transparent barriers, there is a pronounced Andreev reflection feature providing evidence for proximity-induced high-$T_{c}$ superconductivity in 1T-TaS$_{2}$ with a surprisingly large energy gap ($\sim 20\thinspace $meV) equal to half that of intrinsic Bi-2212 ($\sim 40\thinspace $meV). Our systematic study using conductance spectroscopy of junctions with different transparencies also reveals the presence of two separate boson modes, each associated with a "dip-hump" structure. We infer that the proximity-induced high-$T_c$ superconductivity in the 1T-TaS$_2$ is driven by coupling to the metastable metallic phase coexisting within the Mott commensurate CDW (CCDW) phase and associated with a concomitant change of the CCDW order parameter in the interfacial region., Comment: 27 pages, 11 figures

Here we report a quasi-one-dimensional $S=1$ chain compound ${\mathrm{NiTe}}_{2}{\mathrm{O}}_{5}$. From the comprehensive study of the structure and magnetic properties on high-quality single-crystalline ${\mathrm{NiTe}}_{2}{\mathrm{O}}_{5}$, it is revealed that ${\mathrm{NiTe}}_{2}{\mathrm{O}}_{5}$ undergoes a transition into an intriguing long-range antiferromagnetic order at ${T}_{\mathrm{N}}=30.5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, in which longitudinal magnetic moments along the chain direction are ferromagnetically ordered, while their transverse components have an alternating ferromagnetic-antiferromagnetic coupling. Even though the temperature dependence of magnetic susceptibility represents an archetypal anisotropic antiferromagnetic order, we found that critical behavior of unconventional nature with ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\sim}0.25$ and $\ensuremath{\beta}\ensuremath{\sim}0.18$ is accompanied by the temperature evolution of the antiferromagnetic order parameter.

We report studies on the Ba2CuO4- y superconductor (1) with features including compressed local coordination and extreme over-doping level, etc.; while such behaviors are not favorable for superconductivity in previous cuprates (2), Yamamoto (3) raises an inquiry on one of these features: The compressed local octahedron. We have solid evidence to show that what we discussed in our paper has nothing to do with his speculation. There are 2 types of “214” structures with a CuO2 plane, i.e., the T phase based on La2CuO4 and the T′ phase based on Nd2CuO4 due to the different charge reservoir substructures, the rock-salt type for the former and the fluorite type for the latter. The relative strong Coulomb attraction interaction in the rock-salt layer over … [↵][1]1To whom correspondence may be addressed. Email: Jin{at}iphy.ac.cn. [1]: #xref-corresp-1-1

Conventional superconductivity, as used in this review, refers to electron-phonon coupled superconducting electron-pairs described by BCS theory. Unconventional superconductivity refers to superconductors where the Cooper pairs are not bound together by phonon exchange but instead by exchange of some other kind, e. g. spin fluctuations in a superconductor with magnetic order either coexistent or nearby in the phase diagram. Such unconventional superconductivity has been known experimentally since heavy fermion CeCu2Si2, with its strongly correlated 4f electrons, was discovered to superconduct below 0.6 K in 1979. Since the discovery of unconventional superconductivity in the layered cuprates in 1986, the study of these materials saw Tc jump to 164 K by 1994. Further progress in high temperature superconductivity would be aided by understanding the cause of such unconventional pairing. This review compares the fundamental properties of 9 unconventional superconducting classes of materials - from 4f-electron heavy fermions to organic superconductors to classes where only three known members exist to the cuprates with over 200 examples, with the hope that common features will emerge to help theory explain (and predict!) these phenomena. In addition, three new emerging classes of superconductors (topological, interfacial [e. g. FeSe on SrTiO3], and H2S under high pressure) are briefly covered, even though their conventionality is not yet fully determined., accepted for publication in Advances in Physics; comparison of 9 unconventional superconducting classes of materials, plus 3 emerging classes of superconductors whose conventionality is still under discussion; 30 figures, 13 tables, 712 references. This version has minor typographical errors fixed

Although the pairing mechanism of the Fe-based superconductors (FeSCs) has not yet been settled with a consensus, as to the pairing symmetry and the superconducting (SC) gap function, the abundant majority of experiments are supporting for the spin-singlet sign-changing s-wave SC gaps on multibands ($s^{\pm}$-wave state). This multiband $s^{\pm}$-wave state is a very unique gap state {\it per se} and displays numerous unexpected novel SC properties such as a strong reduction of the coherence peak, non-trivial impurity effects, nodal-gap-like nuclear magnetic resonance (NMR) signals, various Volovik effects in the specific heat (SH) and thermal conductivity, and anomalous scaling behaviors with the SH jump and the condensation energy vs. $T_c$, etc. In particular, many of these non-trivial SC properties can be easily mistaken as evidence for a nodal gap state such as a d-wave gap. In this review, we provide detailed explanations of theoretical principles for the various non-trivial SC properties of the $s^{\pm}$-wave pairing state, and then critically compare the theoretical predictions with the experiments of the FeSCs. This will provide a pedagogical overview of how much we can coherently understand the wide range of different experiments of the FeSCs within the $s^{\pm}$-wave gap model., Comment: 82 pages, 51 figures

One of the goals in understanding any new class of superconductors is to search for commonalities with other known superconductors. The present work investigates the superconducting condensation energy, $U$, in the iron based superconductors (IBSs), and compares their $U$ with a broad range of other distinct classes of superconductor, including conventional BCS elements and compounds and the unconventional heavy fermion, $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ru}{\mathrm{O}}_{4},\phantom{\rule{0.16em}{0ex}}\mathrm{L}{\mathrm{i}}_{0.1}\mathrm{ZrNCl},\phantom{\rule{0.16em}{0ex}}\ensuremath{\kappa}{\text{-(BEDT-TTF})}_{2}\mathrm{Cu}(\mathrm{NCS}{)}_{2}$, and optimally doped cuprate superconductors. Surprisingly, both the magnitude and ${T}_{c}$ dependence $(U\ensuremath{\propto}{T}_{c}^{3.4\ifmmode\pm\else\textpm\fi{}0.2})$ of $U$ are---contrary to the previously observed behavior of the specific heat discontinuity at ${T}_{c}$, $\ensuremath{\Delta}C$---quite similar in the IBS and BCS materials for ${T}_{c}g1.4$ K. In contrast, the heavy fermion superconductors' $U$ vs ${T}_{c}$ are strongly (up to a factor of 100) enhanced above the IBS/BCS while the cuprate superconductors' $U$ are strongly (factor of 8) reduced. However, scaling of $U$ with the specific heat \ensuremath{\gamma} (or $\mathrm{\ensuremath{\Delta}}C$) brings all the superconductors investigated onto one universal dependence upon ${T}_{c}$. This apparent universal scaling $U/\phantom{\rule{0.0pt}{0ex}}\ensuremath{\gamma}\ensuremath{\propto}{T}_{c}^{2}$ for all superconductor classes investigated, both weak and strong coupled and both conventional and unconventional, links together extremely disparate behaviors over almost seven orders of magnitude for $U$ and almost three orders of magnitude for ${T}_{c}$. Since $U$ has not yet been explicitly calculated beyond the weak coupling limit, the present results can help direct theoretical efforts into the medium and strong coupling regimes.

Kamihara and coworkers' report of superconductivity at Tc = 26 K in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled 'conventional' superconductors. Clearly superconductivity and magnetism/magnetic fluctuations are intimately related in the FePn/Ch - and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With Tc values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review attempts to provide an overview, using numerous references, with a focus on the materials and their superconductivity., Comment: 30 figures, 4 tables, approximately 600 references; to appear in Rev. Mod. Phys

Specific heat of flux-grown single crystals of YbIr2 in zero and applied magnetic field indicates a more complicated magnetic behavior than was inferred from earlier magnetic susceptibility measurements. The possibility of defect-induced Kondo behavior mixed together with some sort of antiferromagnetic local order is discussed.

We report on X-ray powder diffraction, magnetic susceptibility, electrical resistivity, and specific heat measurements on samples of Ce1-xLaxPt3Si and CePt3Si1+δ in order to better understand this unusual heavy Fermion superconductor’s normal and superconducting state properties. By suppressing the antiferromagnetic transition, TN = 2.2 K, using La-doping on the Ce-site in CePt3Si, we find that in Ce1-xLaxPt3Si, x ≥ 0.8, ρ, χ, and C/T show non-Fermi liquid temperature dependences. C/T ~ Tα with α ~ 1 for the temperature range of 0.3–3 K while low-temperature (2–20 K) susceptibility data follow 1/χ−1/χ0 = aTη behavior with η = 0.36. In addition, χ and C/T exhibit single-ion behavior in this composition range (x ≥ 0.8), i.e. the measured values expressed per Ce-mole are independent of x. Via doping the Si-site with a small excess—2–4—of Si, we present specific heat data for the bulk superconducting transition at Tc ~ 0.8 K showing that this excess sharpens the transition, obviating the need for annealing. Measurements of the X-ray diffraction patterns, ρ, χ, and C/T of these CePt3Si1+δ samples, when compared to similar measurements on annealed CePt3Si samples, indicate that the microscopic mechanism for the strengthening and sharpening of ΔC(Tc) with Si-excess may be similar to that responsible in the annealed material.

Previous research has found that the pairing symmetry in the iron-based superconductor Ba1-xKxFe2As2 changes from nodeless s-wave near optimally doped, x≈0.4-0.55 and Tc>30 K, to nodal (either d-wave or s-wave) at the pure endpoint, x=1 and Tc

The editors have dedicated this special issue on superconducting materials "to Ted Geballe in honor of his numerous seminal contributions to the field of superconducting materials over more than 60 years, on the year of his 95th birthday." Here, as an executive summary, are just a few highlights of his research in superconductivity, leavened with some anecdotes, and ending with some of Teds general insights and words of wisdom., Comment: Contribution to the special issue on Superconductivity and Its Applications in Physica C, volume 514

In the approximately four years since the first review on this subject appeared, a large amount of additional work---both experimental and theoretical---has been accomplished in the field. In an effort to keep the original review experimentally current, the compilation of resistivity, susceptibility, and specific-heat data that was presented in the original ten-page Table II is herein updated with approximately 60 new systems and 70 new references, adding eight new pages to the compilation. It is worth noting that approximately 15% of these new entries in fact predate the original review's publication; certainly the literature searches employed for this update will also have missed work worthy of inclusion.

The question of whether the temperature dependences of the magnetic susceptibility and the electrical resistivity of UCu5−xNix near, and away from, the QCP where TN is suppressed to T = 0 are due to intrinsic fluctuations or are dominated by disorder effects is addressed. The interesting ρ ∝ log T behavior below 2 K present for 0.75 ⩽ x ⩽ 1.1 is analyzed and discussed.

We measure the heat capacity of a UBe13 sample with an unusually low T c for a polycrystal. We find an upturn in the upper critical field H c2(T) below about T c/2, much as for higher-T c samples. Comparing the critical fields in our sample and in samples with higher T c's shows that the low temperature limit of H c2 is proportional to T c(H = 0), as expected if the upturn comes from an FFLO phase and strong coupling.

Recently Takeuchi reported a weak, 'metamagnetic-like' increase in the magnetization around 42 T in single crystals of CeIrIn 5 for field in the c-direction. We report specific heat measurements on single crystal CeIrIn 5, H parallel c-axis, measured in the dc hybrid magnet at NHMFL in Tallahassee between 1.4 and 10 K. A clear anomaly in C/T in 35 T is observed to peak at 1.8 K, with an entropy of 6% of RIn2. This anomaly grows in size and shifts upwards in temperature (both monotonically) with increasing H until at 45 T T peak =4.1 K , with the entropy associated with the transition equal to 14% of RIn2. C/T data at 32 T show3 only the high temperature side of the peak occurring above 1.4 K, while C/T data at 28.5 T show no anomaly down to 1.4 K. This is consistent with our Tpeak vs H data, which imply T peak → 0 at 26 T.

We report on recent magnetic field studies on U1−xThxBe13 with x≤0.1. These new results are related to previous data published by Kromer et al. [F. Kromer et al., Phys. Rev. B 62, 12477 (2000)]. The new magnetization measurements confirm the existence of an anomaly found earlier through calorimetric and dilatometric measurements at TL

The heavy-fermion system CePtSi is located close to a magnetic instability. In order to study the influence of composition on the physical properties of CePtSi we prepared slightly off-stoichiometric CePt1+xSi1−x polycrystals. CePt1+xSi1−x with x=−0.04 exhibits typical non-Fermi-liquid behavior (C/T∝lnT, χ∝T−0.21, ρ∝T1.5). A scaling analysis of the thermodynamic quantities at magnetic fields up to 10T and temperatures up to 6K points to a quantum phase transition described by some unknown non-Gaussian fixed point. In contrast substitution of Si by Ge leads to a quantum phase transition involving weak-coupling spin fluctuations for CePtSi0.89Ge0.11, which can be described within the framework of the self-consistent renormalization theory.

Heavy Fermion physics deals with the ground state formation and interactions in f-electron materials where the electron effective masses are extremely large, more than 100 times the rest mass of an electron. The details of how the f-electrons correlate at low temperature to become so massive lacks a coherent theory, partially because so few materials display this heavy behavior and thus global trends remain unclear. UHg_{3} is now found experimentally to be a heavy Fermion antiferromagnet, just as are all the other U_{x}M_{y} compounds with the metal M being in column II B (filled d electron shells) in the periodic table (Zn/Cd/Hg) and the spacing between Uranium ions being greater than the Hill limit of 3.5 Angstroms. This result that, independent of the structure of these U_{x}M_{y}, M=Zn/Cd/Hg, compounds and independent of the value of their Uranium Uranium spacings (ranging from 4.39 to 6.56 Angstroms), all exhibit heavy Fermion antiferromagnetism, is a clear narrowing of the parameters important for understanding the formation of this ground state. The sequence of antiferromagnetic transition temperatures, T_{Neel}, of 9.7 K, 5.0 K, and 2.6 K for U_{x}M_{y} as the metal M varies down column II B (Zn/Cd/Hg) indicates an interesting regularity for the antiferromagnetic coupling strength.

A relatively new class of materials has been found in which the basic assumption of Landau Fermi-liquid theory---that at low energies the electrons in a metal should behave essentially as a collection of weakly interacting particles---is violated. These ``non-Fermi-liquid'' systems exhibit unusual temperature dependences in their low-temperature properties, including several examples in which the specific heat divided by temperature shows a singular $\mathrm{log}T$ temperature dependence over more than two orders of magnitude, from the lowest measured temperatures in the milliKelvin regime to temperatures over 10 K. These anomalous properties, with their often pure power-law or logarithmic temperature dependences over broad temperature ranges and inherent low characteristic energies, have attracted active theoretical interest from the first experimental report in 1991. This article first describes the various theoretical approaches to trying to understand the source of strong temperature- and frequency-dependent electron-electron interactions in non-Fermi-liquid systems. It then discusses the current experimental body of knowledge, including a compilation of data on non-Fermi-liquid behavior in over 50 systems. The disparate data reveal some interesting correlations and trends and serve to point up a number of areas where further theoretical and experimental work is needed.

Investigation of the low-temperature specific heat, magnetic susceptibility, and resistivity of ${\mathrm{U}}_{2}{\mathrm{Co}}_{2}\mathrm{Sn}$ for expected paramagnon behavior led instead to the discovery of a non-Fermi-liquid system obeying the Millis/Moriya theory prediction of an electronic specific heat that varies approximately as $\ensuremath{\gamma}\ensuremath{-}A\sqrt{T}$ over an appreciable temperature range, in this case over the whole temperature range (0.3--10 K) of measurement. The temperature dependence of the low-temperature resistivity, however, follows $\ensuremath{\rho}={\ensuremath{\rho}}_{0}{+AT}^{\ensuremath{\alpha}},$ with $\ensuremath{\alpha}(\ensuremath{\approx}1.8)$ lying below that predicted for a Fermi liquid (i.e., $\ensuremath{\alpha}=2)$ but above the $\ensuremath{\rho}={\ensuremath{\rho}}_{0}{+AT}^{1.5}$ predicted by the quantum phase transition, weakly interacting spin fluctuation theory of Millis/Moriya. Scaling of the specific heat with field indicates that the electron interactions responsible for the non-Fermi-liquid behavior are not single ion in nature. Several non-Fermi-liquid theories and their possible applicability to these results are discussed, as well as the possible influence of the relatively small U-U spacing in ${\mathrm{U}}_{2}{\mathrm{Co}}_{2}\mathrm{Sn}$ on the unusual non-Fermi-liquid behavior.

We report specific heat measurements in a broad temperature range between 0.06 and 20 K on single crystalline CeRu2Si2 in magnetic fields applied parallel to the c-direction around the critical field B M , where the metamagnetic transition from the itinerant to the localized state takes place. In the critical field B M ∥ c=7.8 T a distinct deviation from the usual Fermi- liquid behavior is found down to our lowest temperature. Below 1.8 K C/T varies as (1−aT), while for 1.8 K

Scaling of low-temperature magnetization data for the non-Fermi-liquid systems ${\mathrm{UCu}}_{4}\mathrm{Pd}$ and ${\mathrm{UCu}}_{3.5}{\mathrm{Pd}}_{1.5}$ in fields to 30 T is carried out to help determine the nature of the magnetic interactions above and below 10 K. The size of the scaling exponent \ensuremath{\beta} found from this scaling with fields above 10 K implies that the higher-temperature magnetic excitations in ${\mathrm{UCu}}_{4}\mathrm{Pd}$ are correlated, i.e., $\ensuremath{\beta}g1.0.$ For $Tl~6\mathrm{K},$ $\ensuremath{\beta}l1.0$ for both ${\mathrm{UCu}}_{4}\mathrm{Pd}$ and ${\mathrm{UCu}}_{3.5}{\mathrm{Pd}}_{1.5}$ implying that the excitations may change and assume single-ion character, consistent with recent doping experiments. These results, which contradict a recent neutron-scattering study, may be understood theoretically as a depletion of the occupation of the phase space of magnetic excitations with decreasing temperature ( energy), leading to a decoupling of the excitations.

We report on low-temperature thermal-expansion and specific-heat measurements on single crystals of the heavy-fermion superconductor UBe13. A new “line of anomalies”, B*(T), in the superconducting state is discovered. The evolution of this feature is studied as a function of Th doping on several U1−xThxBe13 polycrystals (x⩽0.03), giving rise to a new “line of anomalies”, TL(x), in the T–x phase diagram. TL(x) hits the multi-critical point at xcr≈0.019 and merges smoothly into the Tc2 line that marks the lower of the two phase transitions at x>xcr. Our data strongly suggest that the anomaly at TL(x) represents the precursor of the transition at Tc2(x) for 0.019

We have carried out transverse-field ( H = 1.5 T ) μ SR experiments in UCu 4 Ni for temperatures from 300 down to 2.2 K. The muon-decay asymmetry can be fit to a stretched exponential relaxation function: G ( t ) ∝ e - ( Λ t ) K . The exponent K ∼ 2 for T > 100 K , but drops continuously below this temperature (being ∼ 1 for T ∼ 20 K and reaching ∼ 0.6 near 2 K). The characteristic relaxation rate Λ grows six-fold in the experimental temperature range (from ∼ 0.2 for T ∼ 300 K to ∼ 1.2 μ s - 1 for T ∼ 2 K ). Independently of theoretical models, the behavior of these parameters is consistent with strong magnetic disorder. Although the magnetic susceptibility of UCu 4 Ni is considerably lower than that of UCu 4 Pd , the amount of magnetic disorder in both systems is comparable. We discuss the data within the context of the Kondo disorder model.

Low-temperature specific-heat and thermal-expansion measurements on a single crystal of the heavy-fermion superconductor ${\mathrm{UBe}}_{13}$ reveal an anomaly in the superconducting state. The evolution of this feature is studied as a function of Th doping on several ${\mathrm{U}}_{1\ensuremath{-}x}{\mathrm{Th}}_{x}{\mathrm{Be}}_{13}$ polycrystals ( $x\ensuremath{\le}0.03$), giving rise to a new ``line of anomalies,'' ${T}_{L}\left(x\right)$, in the $T\ensuremath{-}x$ diagram. The anomaly at ${T}_{L}(x)$ marks the precursor of the lower of the two phase transitions at $xg{x}_{\mathrm{cr}}\ensuremath{\approx}0.019$. Implications for the interpretation of the phase diagram of superconducting ${\mathrm{U}}_{1\ensuremath{-}x}{\mathrm{Th}}_{x}{\mathrm{Be}}_{13}$ are addressed.

Specific-heat measurements have been carried out to further elucidate the electronic and magnetic properties of ${\mathrm{Ce}}_{8}{\mathrm{Pd}}_{24}M$ ($M$=Ga, In, Sn, Sb, Pb, and Bi). The measurements show that a slight change in the chemical and electronic structure of ${\mathrm{CePd}}_{3}$, by forming the ordered ternary ${\mathrm{Ce}}_{8}{\mathrm{Pd}}_{24}M$-type compounds, induces an enhanced low-temperature electronic specific heat \ensuremath{\gamma}. This indicates the emergence of heavy-fermionic behavior with the loss of the intermediate valence nature of ${\mathrm{CePd}}_{3}$. All of the ${\mathrm{Ce}}_{8}{\mathrm{Pd}}_{24}M$-type compounds studied here exhibit a sharp peak in the specific heat at low temperature due to antiferromagnetic (AF) transitions. Below the transition temperature, the specific heat shows the heavy-fermionic behavior, i.e., \ensuremath{\gamma}( $T$\ensuremath{\rightarrow}0) equal to a finite value, rather than \ensuremath{\gamma}( $T$\ensuremath{\rightarrow}0)=0 in the paramagnetic AF ground state.

Break junctions of the heavy-fermion superconductors CeCu 2 Si 2 , UBe 13 , UPt 3 , URu 2 Si 2 , UPd 2 Al 3 , and UNi 2 Al 3 have been investigated. Josephson-like superconducting anomalies could be found only for low-ohmic contacts, but without the oscillatory Fraunhofer pattern of the critical current in a magnetic field. A systematic study on the contact radius demonstrates that these anomalies are mainly due to Maxwell's resistance being suppressed in the superconducting heavy-fermion phase rather than due to Josephson effect and Andreev reflection. We could not find any superconducting features by vacuum-tunneling spectroscopy.

Spectroscopic, magnetic, thermal, and transport measurements under pressure are presented which were performed on ${\mathrm{YbCu}}_{5\ensuremath{-}x}{\mathrm{Al}}_{x}$ intermetallics. The substitution of Cu by Al in ${\mathrm{YbCu}}_{5}$ drives the system from an almost divalent behavior of the Yb ion in ${\mathrm{YbCu}}_{5}$ to a trivalent state in ${\mathrm{YbCu}}_{3}{\mathrm{Al}}_{2}$. A compensation between the normal thermal expansion and a valence-driven contraction was found for ${\mathrm{YbCu}}_{4}\mathrm{Al}$. Antiferromagnetic order sets in for $xg1.5$, where an increasing Al content causes both a growing transition temperature and rising saturation moments reaching 2 K and about 2.1${\ensuremath{\mu}}_{\mathrm{B}},$ respectively, for $x=2$. At the critical concentration $(x\ensuremath{\approx}1.5)$ the possibility of a non-Fermi-liquid state is analyzed together with its tuning by pressure.

Th{sub 1{minus}x}U{sub x}Cu{sub 2}Si{sub 2} compounds have been investigated by means of specific heat, susceptibility, magnetization, and resistivity measurements. Samples with x{ge}0.15 are ferromagnetic. Th{sub 0.85}U{sub 0.15}Cu{sub 2}Si{sub 2} and Th{sub 0.8}U{sub 0.2}Cu{sub 2}Si{sub 2} show enhanced values for C/T at low temperatures ({approx}460 mJ/UmolK{sup 2} for x=0.2). The specific heat of samples that exhibit no long-range order (x{le}0.1) can be described by C/T{proportional_to}{minus}ln(T). This divergence is characteristic of non-Fermi-liquid (nFl) systems. The results are discussed in regard to the current investigations on non-Fermi liquids. One interpretation of non-Fermi-liquid behavior is a T=0 instability in nearness to magnetism. Our data indicate that this is a system that displays C/T{proportional_to}lnT near to ferromagnetism. {copyright} {ital 1997} {ital The American Physical Society}

Using the canonical spin-fluctuation system UAl{sub 2} as a starting point, via negative chemical pressure (doping with Y) we have expanded d{sub U-U} in a system known to be near the Hill limit of f-electron localization, and characterized the samples via resistivity, magnetic susceptibility, and specific-heat measurements. All system parameters, including magnetic susceptibility, specific heat {gamma} ({equivalent_to}C/Tlim{sub T{r_arrow}0}), and spin-fluctuation temperature, behave monotonically. For U{sub 1{minus}x}Y{sub x}Al{sub 2}, 0.30{le}x{le}0.70, spin-glass behavior is found with T{sub f}{approx_equal};5.1{plus_minus}0.5 K. This spin-glass behavior weakens (T{sub f} sinks, smaller magnetic signature, no specific-heat anomaly) for x{ge}0.75 while, at the same time, the spin-fluctuation T{sup 3}lnT term also gradually disappears from the specific heat. For x{ge}0.875, a non-Fermi-liquid (nFl) lnT term is found in the low temperature C/T. This new, perhaps equilibrium, ground state persists upon further dilution of the U ions with Y. Thus, we report on the evolution of nFl behavior in the neighborhood of a spin-glass ground state but, indeed, directly out of a yet weaker form of magnetism than heretofore reported, that of spin fluctuations. {copyright} {ital 1997} {ital The American Physical Society}

We present low-temperature thermal expansion measurements on the heavy fermion system CeCu 5.8 Ag 0.2 , which is located at an antiferromagnetic (AF) quantum critical point (QCP). At zero magnetic field, the volume expansion coefficient divided by temperature shows a logarithmic divergence upon cooling below 1 K. This temperature dependence is incompatible with the predictions of the itinerant spin-density wave theory for an AF QCP. The application of magnetic fields leads to a cross-over to Landau Fermi liquid behavior as expected for a zero-field QCP.

The resistivity vs temperature of annealed and unannealed single crystals of Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ over a broad range of nominal compositions between $x$ $=$ 0.05 and 0.30, with special focus on compositions around that (${x}_{\mathrm{opt}}$) of the optimal superconducting transition temperature, is reported. Above any transition temperature, either superconducting (${T}_{c}$) or structural (${T}_{S}$) for $x$ ${x}_{\mathrm{opt}}$, the low temperature resistivity, \ensuremath{\rho}, behaves approximately linearly with temperature from $x$ $=$ 0.05 up to at least $x$ $=$ 0.095, with $\ensuremath{\rho}\ensuremath{\propto}{T}^{2}$ starting for $x$ $=$ 0.14. The apparent constancy of the temperature dependence over a broad composition range argues strongly for the presence of a quantum critical point (QCP) underneath the superconducting dome. However, the temperature dependences measured down to ${T}_{c}$ leave the exact location of the QCP open. Interestingly, although the superconducting transition temperature rises by \ensuremath{\approx}1.5 K with annealing, the temperature dependence of \ensuremath{\rho} appears unaffected by annealing, i.e., the QCP appears insensitive to small amounts of lattice disorder.

The complex microwave impedance in the frequency range of ≈ 8–36 GHz was measured in the superconducting state of a well-characterized heavy fermion single crystal of UBe 13 . This microwave photon energy ranges roughly from 15% to 60% of the zero-temperature (BCS) energy gap and is higher than previous measurements. Large changes are observed in the complex impedance when the sample is cooled below T c for all frequencies measured. The surface impedance cannot be described by the Mattis-Bardeen theory in the dirty limit for a conventional superconductor.

We have discovered, in contrast with most dopants {ital M}, that Hf (Zr) can be doped to quite high levels in U{sub 1{minus}{ital x}}M{sub {ital x}}Pt{sub 3}, up to {ital x}=0.15 (0.30). Above 5{percent} doping, the temperature dependence of the low-temperature upturn in {ital C}/{ital T} (specific heat divided by temperature) changes from {ital T}{sup 3}ln{ital T} (indicative of spin fluctuations) to {ital T}ln{ital T} [indicative of non-Fermi-liquid (NFL) behavior]. The specific heat NFL temperature dependence has been measured down to 0.1 K and is seen over more than two decades in temperature. The field dependence of the NFL specific heat is small, consistent with a quadrupolar moment model interpretation. A possible reason for the NFL behavior is discussed. {copyright} {ital 1996 The American Physical Society.}

We present the results of specific-heat and resistivity measurements as a function of temperature, magnetic field and hydrostatic pressure on the Kondo lattice CeNi2Ge2, the heavy-fermion superconductors CeCu2Si2 and UBe13 as well as the low-carrier-density system Yb4As3. “Non-Fermi-liquid” effects in the low-temperature normalstate properties of the three former systems are consistent with the existence of a “nearby” quantum critical point, presumably of antiferromagnetic type. Yb4As3, though showing the outward appearance of a Landau-type heavy-fermion metal, behaves very differently, i.e. as an extreme two-fluid system.

The upper critical fieldH c2 (T) of a high quality single crystal of UBe13 is studied with very low noise resistivity measurements. It shows a large but finite slope of —45 T/K, an unusual temperature dependence with an inflexion point atT/T C ∼ 0.5 and a large saturated limit forT→ 0 ofH c2 (0) = 14 T. The complete temperature dependence ofH c2 (T) can be described by a simple model of strong coupling superconductor, assuming a full Pauli limitation and the occurence of a non-uniform superconducting state (FFLO state) at low temperature.

Using sensitive mutual induction techniques, we have measured the real part of the susceptibility, {chi}{prime}({ital B},{ital T}), and the imaginary part, {chi}{prime}{prime}({ital B},{ital T}), for three UBe{sub 13} single crystals below 1 K and in fields up to 8 T oriented along the [100] and [110] directions, with particular emphasis on the region near the zero-field critical temperature {ital T}{sub {ital c}}. In this regime and within our experimental uncertainties of 0.5 mK and 5 mT, no anisotropy was observable for any of the samples. For all three samples, the magnetic susceptibility between 4 and 15 K was measured and subtle features were found for two of the samples around 10 K. For one of these specimens, an anomaly in {chi}{prime}({ital B},{ital T}) was observed below the superconducting transition and is likely due to flux pinning mechanisms. The {ital B}{sub {ital c}2}({ital T}) phase diagrams are presented and compared to the results of other workers. Finally, the temperature dependence of the penetration depth for 0.1{le}{ital T}/{ital T}{sub {ital c}}{le}0.5 was found to be close to {ital T}{sup 3}, independent of field orientation, and these data are presented and discussed.

Doping UBe{sub 13} with Th, La, Pr, or Ce destroys the as-yet not understood anomaly in the specific heat at 2 K. Using U{sub 0.97}Th{sub 0.03}Be{sub 13} as a background, we note that the anomaly in UBe{sub 13} fits very well with a Schottky temperature dependence, with fit parameters centered around a level splitting of 7 K but with only {similar_to}5% of the amplitude expected if all of the U ions participated. The decrease observed in the resistivity in UBe{sub 13} below 2 K is consistent with the freezing out of some type of degrees of freedom, which could also include magnetic scattering. Based on the known lack of magnetic-field dependence of the magnetization and the specific heat in the UBe{sub 13}, however, these low-lying {ital f} states cannot simply be understood as noncompensated magnetic moments as observed in, e.g., CeBe{sub 13}. We infer that this anomaly is intrinsic based on its behavior with annealing and improved purity, and on the fact that the entropy under this Schottky anomaly is shifted upon doping into more than a factor of 2 increase in the low-temperature {ital m}{sup *}({proportional_to}{gamma}) in U{sub 0.97}Th{sub 0.03}Be{sub 13}. The occurrence of a similar anomaly in anothermore » heavy-fermion superconductor, CeCu{sub 2}Si{sub 2}, makes the anomaly of some interest.« less

We compare the local magnetization at a spatial singularity of superconducting U_(0.97)Th_(0.03)Be_(13) for different magnetic field histories. We find a reduction in the zero-field-cooled vortex flow relative to the field-cooled case, marked by a sharp onset coincident in temperature with the second (lower-temperature) transition in the specific heat. This effect is peculiar to the flux threading the hole of the torus, to (U,Th)Be_(13) samples with multiple superconducting transitions, and to the lower superconducting state with broken time-reversal invariance.

The strong power law behavior of the specific heat jump $${\rm{\Delta }}C\;$$ versus T c $$({\rm{\Delta }}C/{T}_{ {\rm{c}}}\sim {T}_{ {\rm{c}}}^{\alpha },\alpha \approx 2)$$, first observed by Bud'ko et al (2009 Phys. Rev. B 79 220516), has been confirmed with several families of the Fe-based superconducting compounds with various dopings. We tested a minimal two band BCS model to understand this anomalous behavior and showed that this non-BCS relation between $${\rm{\Delta }}C\;$$ versus T c is a generic property of the multiband superconducting state paired by a dominant interband interaction ($${V}_{\mathrm{inter}}\gt {V}_{\mathrm{intra}}$$) reflecting the relation $$\frac{ { {\rm{\Delta }}}_{ {\rm{h}}}}{ { {\rm{\Delta }}}_{ {\rm{e}}}}\sim \sqrt{\frac{ {N}_{ {\rm{e}}}}{ {N}_{ {\rm{h}}}}}$$ near T c, as in the $${S}_{\pm }$$-wave pairing state. We also found that this $${\rm{\Delta }}C\;$$ versus T c power law can continuously change from the ideal BNC scaling to a considerable deviation by a moderate variation of the impurity scattering rate $${ {\rm{\Gamma }}}_{0}$$ (non-pair-breaking). Finally, as a result, our model provides a consistent explanation why the electron-doped Fe-based superconductors follow the ideal BNC scaling very well while the hole-doped systems often show varying degree of deviations.