Search Results (181 total)

We have investigated the effect of size on the specific heat and magnetic susceptibility of heavy Fermion Ce(Ru_1−xRh_x)₂Si₂ at x=0.6, which is a composition near a quantum critical point. Samples in the form of pellets pressed from powders ranging over two decades in size from d∼0.6–1.2 μm up to 53–120 μm were investigated. Size was characterized via sieving (d>20 μm) or filtration (d≤20 μm) through a series of decreasing size mesh or pores, by the line broadening of high-angle x-ray lines, as well as scanning electron microscopy measurements on the smallest particles. The magnetic susceptibility, χ, at low temperature increases strongly at the smallest sizes, reaching approximately a factor of 5 increase for the smallest powders vs the starting bulk material χ value. The magnitude and temperature dependence of the low-temperature specific heat, C, (C/T∼log T at a quantum critical point) down to 0.15 K remains essentially unchanged with size reduction down to 3 μm. Below 3 μm, however, a new regime is entered. C/T at low temperatures begins to show a steep increase, i.e., it becomes more divergent, above the log T behavior, with C/T∼γ+aT^−0.67 over more than two decades of temperature down to 0.1 K. This altered non-Fermi-liquid temperature dependence is consistent with the low-temperature behavior of χ and with the field dependence of the magnetization. Together, these emergent properties at the verge of the nanosize regime are reminiscent of Griffiths phase (rare spin cluster) behavior. Thus, decreasing the size down to ∼1 μm does not reveal size limitation of the infinite-range fluctuations expected at this quantum critical point. Instead, strain and defects inherent in the small size appear to produce rare spin cluster-dominated effects as d→1–3 μm, with uncompensated local-moment defects becoming more dominant as size reaches the nanoregime d∼0.6–1.2 μm—consistent with the previous Kondo-dominated results on 20-nm-sized Ce compounds. Whether such rare spin cluster effects would also occur away from the quantum critical concentration is discussed.

Measurements of magnetotransport and current-voltage (I-V) characteristics up to 9 T were used to investigate the vortex phase diagram of an underdoped (Ba,K)Fe₂As₂ single crystal with T_c=26.2 K. It is found that the anisotropy ratio of the upper critical field H_c2 decreases from 4 to 2.8 with decreasing temperature from T_c to 24.8 K. Consistent with the vortex-glass theory, the I-V curves measured at H=9 T can be well scaled with the vortex-glass transition temperature of T_g=20.7 K and critical exponents z=4.1 and ν=1. Analyses in different magnetic fields produced almost identical critical exponent values, with some variation in T_g, corroborating the existence of the vortex-glass transition in this underdoped (Ba,K)Fe₂As₂ single crystal up to 9 T. A vortex phase diagram is presented, based on the evolution of T_g and H_c2 with magnetic field.

Specific heat has been measured down to 0.053 K on a single crystal of the heavy-fermion antiferromagnet U₃Ni₅Al₁₉ that orders at T_N=23 K. As has been previously reported, these data can be fitted between 0.4 and 4 K by the spin-fluctuation model of Moriya and Takimoto, which describes the contribution of weakly interacting critical spin fluctuations to the specific heat, C, where, as T→0, C/T=γ₀−a√T. However, below 0.35 K a noticeable divergence in C/T∼log T dependence, consistent with the existence of strongly interacting fluctuations, is observed. This increase in the divergence of C/T at the lowest temperatures—which is contrary to the self-consistent renormalization theory of Moriya and Takimoto, which predicts √T dependence for C/T as T→0 and log T dependence at higher temperatures—has been measured as a function of magnetic field to further understand its origin. The field data in the low-temperature regime, where C/T∼log T exhibit scaling with ΔB/T^1.9, further evidence that there exist strongly interacting fluctuations below 0.35 K in U₃Ni₅Al₁₉.

Coexistent magnetism and non-Fermi-liquid (nFl) behavior occur in only a few of the more than 100 known nFl systems. We have investigated the resistivity, dc magnetic susceptibility, and specific heat as a function of field (0–13 T) down to 0.4 K (0.09 K for resistivity) in high-purity polycrystalline U(Pt_0.94Pd_0.06)₃, where antiferromagnetism at 6 K is known to precede non-Fermi-liquid behavior in the specific heat at lower temperatures in zero field. Unusually for a system that shows nFl behavior in the bulk specific heat, both the dc magnetic susceptibility χ and the electrical resistivity ρ of U(Pt_0.94Pd_0.06)₃ show Fermi-liquid behavior down to our lowest temperature of measurement. Possible inferences about the q dependence of the non-Fermi-liquid fluctuation spectrum are discussed. As expected, magnetic field succeeds in driving the specific heat of the system back into a Fermi-liquid ground state with C/T saturating to a constant value below about 0.9 K.

By partially replacing the U in single crystals of UIr₂Zn₂₀ with Th, we have suppressed the 2.1 K ferromagnetic-like transition present in the pure compound. The magnetic susceptibilities of U_1−xTh_xIr₂Zn₂₀ show enhanced low-temperature values, with χ  (2 K) for x=0.25 around 140 memu/Umole. However, unlike the parent compound, for U_1−xTh_xIr₂Zn₂₀ (x≥0.25) the extrapolation of even the lowest temperature 1/χ vs T data shows an intercept on the negative temperature axis—consistent with antiferromagnetic fluctuations. The specific heat at low temperature in U_0.75Th_0.25Ir₂Zn₂₀ shows no magnetic transition down to 0.4 K, but rather obeys C/T=γ−aT^0.5 over more than a decade of temperature, consistent with weak-coupling three-dimensional antiferromagnetic fluctuations. Low temperature resistivity data show an unusual non-Fermi-liquid temperature dependence, where ρ=ρ₀+aT^α with α<1. The specific-heat data for U_0.5Th_0.5Ir₂Zn₂₀ can be fit within the normal Fermi-liquid picture by the addition of a T³ log T/T_SF term, where T_SF is the spin-fluctuation temperature.

The force concept inventory and a 10-question context-modified test were given to 647 students enrolled in introductory physics classes at the University of Arkansas. Context changes had an effect ranging from −3% to 10% on the individual questions. The average student score on the ten transformed questions was 3% higher than the average student score on the corresponding 10 force concept inventory questions. Therefore, the effect of contextual changes on the total of the 10 questions is not sufficient to affect normal use of the force concept inventory as a diagnostic instrument.

The magnetic phase diagram of Ce(Ru_1−xRh_x)₂Si₂ contains a spin-density-wave (SDW) magnetic ordering temperature approaching T=0 at both x≈0.03 and 0.35–0.4 (i.e., a dome-shaped phase boundary) and long range, local moment antiferromagnetism, T_N=36 K, in pure CeRh₂Si₂ suppressed with Ru doping to T=0 at x≈0.6–0.65. This latter possible second quantum critical point (QCP), and the possible interplay between the fluctuations caused at each of the two QCP’s, are investigated here using specific heat, resistivity, and dc-magnetic susceptibility data over a broad range of composition, from x=0.04 to 0.8. One principal result is that the specific heat divided by temperature, C∕T, for x=0.6 (0.65) near T_{local moment}→0 is proportional to log T over 2 1∕2 decades of temperature down to the lowest temperature of measurement, 0.04 (0.3) K, indicative of strong fluctuations near a QCP. For the region in the phase diagram x=0.4–0.5, i.e., near the reported T_SDW→0 composition, C∕T measured down to 0.08 K in the present work, as well as literature data, show a distinctly different behavior with temperature, a saturation in C∕T which can be fit by γ−asqrt[T]. Such a temperature dependence is consistent with a nearby QCP with weakly interacting spin fluctuations as proposed, e.g., in the theory of Moriya. In the region between the two QCP’s, at x=0.55, specific heat data down to 0.1 K are not well fit by C∕T=γ−asqrt[T] and are consistent with C∕T∼log T only down to 0.6 K, i.e., this composition displays intermediate behavior. The residual resistivity, ρ₀, vs x shows two strong peaks, at x=0.4 and 0.65, consistent with the existence of two quantum critical points. The exponent α in ρ=ρ₀+AT^α indicates non-Fermi liquid behavior, with α varying monotonically—in contrast to ρ₀—from 1.5 to ∼0.9 between x=0.3 and 0.65. The Fermi liquid exponent, α=2, is recovered at x=0.8. These results taken together indicate two distinct quantum critical points in the phase diagram of Ce(Ru_1−xRh_x)₂Si₂, with different fluctuation strengths at x (T_SDW→0)=0.4 and x(T_{local moment}→0)≈0.6–0.65.

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.

Using specific heat down to 0.05 K, we determine the variation of the non-Fermi liquid behavior in strongly disordered UCu_5−xNi_x, 0.5⩽x⩽1.2, as the Ni doping is varied through the quantum critical concentration, x_QC, that suppresses the 16.5 K antiferromagnetism in pure UCu₅. Contrary to the case of Pd-doped UCu₅, where the Pd experiences partial sublattice ordering, Ni—which unlike Pd is smaller than Cu—goes equally on the four equivalent smaller Cu sites in UCu₅ at a concentration of about 25% at x_QC=1. This ensures strong lattice disorder. This disorder is confirmed by large high angle x-ray linewidths and large measured residual resistivity values of ⩾700 μΩ cm. C∕T≈log T over the whole temperature range of measurement in the well-disordered UCu_5−xNi_x only within 5% of x_QC. By x=1.1, a temperature dependence for C∕T sets in below 0.3 K that is more divergent than either the log T or T^−1+λ fits to the higher temperature data, similar to behavior recently observed and analyzed in YbRh₂Si₂. This progression of the temperature dependence of the specific heat in UCu_5−xNi_x around x_QC≈1 as a function of x is rapid compared to other known doped non-Fermi liquid systems.

PuGa₃ crystallizes in either a trigonal structure type (R-3m) or in the hexagonal DO19 type (P6₃∕mmc). The magnetic properties of both allotropes were investigated by magnetization, specific heat, and electrical resistivity measurements down to low temperatures and under magnetic fields and high pressures. Both phases order magnetically; the trigonal modification corresponds to a soft ferromagnet below T_C=20 K with a saturated moment of 0.2 μ_B∕Pu, whereas the hexagonal one exhibits antiferromagnetic order below T_N=24 K which undergoes a metamagnetic transition at μ₀H_M≈6.1 T (at T=5 K). In the paramagnetic state, the effective magnetic moment inferred from a modified Curie-Weiss law amounts to μ_eff≈0.78μ_B for both phases, indicative of the occurrence of a Pu³⁺ charge state. The values of the electronic specific heat coefficient γ≈110 and 220 mJ∕mol K² for the trigonal and hexagonal allotropes, respectively, indicate a moderate heavy fermion character. Comparisons with related compounds PuCoGa₅, UGa₃, and NpGa₃ suggest a strong tendency toward 5f delocalization.

Specific heat studies have shown a distinct second transition at T≈0.48 K in some nominally single-phase samples of the heavy Fermion superconductor CePt₃Si, T_c=0.75 K, which in the current work is also seen as structure in penetration depth measurements. The double transition structure is accentuated by a Ce deficiency in Ce_1−xPt₃Si, becoming reproducible for x⩾2%. Electron microprobe, x-ray diffraction, and metallography studies of these Ce-deficient samples show only a minor amount of second phase which, by secondary electron backscattering, is similar in composition to Ce₂Pt₁₅Si₇. This compound is an antiferromagnet with a peak in C∕T at 0.4 K with a magnitude of 22000 mJ (Ce-mol K²)⁻¹. Thus, in the complexity of a the Ce-Pt-Si ternary phase diagram, the cause for the occasionally observed structure in the specific heat below the superconducting transition is a few tenths of a percent mass loss of Ce during arc-melting or annealing of the sample.

²⁹Si nuclear spin-lattice relaxation measurements have been performed in the heavy-fermion alloys CePtSi_1−xGe_x, x=0 and 0.1, in order to study spin dynamics near a magnetic instability. The spin-relaxation curves for both x=0 and x=0.1 are found to fit to a stretched exponential slightly better than a single exponential function, which suggests that the relaxation rates are inhomogeneous to some extent, due to structural disorder. Within experimental resolution, the relaxation curve is in agreement with the theoretical curve calculated from the Kondo-disorder model. The temperature-dependent relaxation rate 1∕T₁ follows the Korringa relation in CePtSi below 4 K, but not in CePtSi_0.9Ge_0.1. This means that Fermi-liquid and non-Fermi-liquid excitations appear in x=0 and x=0.1 samples, respectively, which agrees with the results from specific heat experiments. The effective moments of Ce³⁺ ions in CePtSi_1−xGe_x for directions parallel and perpendicular to the c axis, calculated from the crystalline electric field (CEF) levels, explain the anisotropy in the observed susceptibilities, hyperfine coupling constants, and spin-lattice relaxation rates. The magnitude of the CEF-corrected Korringa products for the two samples shows the same order as the expected value for a Fermi gas, which indicates that no obvious spin-correlated fluctuations or magnetic order are present. This seems to disagree with the Griffiths phase model, in which magnetic clusters are spatially-extended objects containing many spins.

The superconducting properties of the recently discovered PuMGa5 (M=Co,Rh) superconductors, including the power law behavior of the specific heat, the evolution of the superconducting transition T_c temperature with pressure, and the linear relation between T_c and ratio of tetragonal lattice parameters c/a, are compared to those of the heavy fermion CeMIn5 (M=Co,Rh,Ir) unconventional superconductors. The striking similarity of the properties between the two families of superconductors suggests a common physics and a common (magnetically mediated) mechanism of superconductivity.

The heavy-fermion system CeCu_6-xAg_x is studied at its antiferromagnetic quantum critical point, x_c=0.2, by low-temperature (T≥50  mK) specific heat, C(T), and volume thermal expansion, β(T), measurements. Whereas C/T∝log⁡(T₀/T) would be compatible with the predictions of the itinerant spin-density-wave (SDW) theory for two-dimensional critical spin fluctuations, β(T)/T and the Grüneisen ratio, Γ(T)∝β/C, diverge much weaker than expected, in strong contrast to this model. Both C and β, plotted as a function of the reduced temperature t=T/T₀ with T₀=4.6  K, are similar to what was observed for YbRh₂(Si_0.95Ge_0.05)₂ (T₀=23.3  K), indicating a striking discrepancy to the SDW prediction in both systems.

The magnetic susceptibility and nuclear magnetic resonance (NMR) linewidth have been measured in the heavy-fermion alloys CePtSi_1−xGe_x, x=0 and 0.1, to study the role of disorder in the non-Fermi-liquid (NFL) behavior of this system. The theoretical NMR line shape is calculated from disorder-driven NFL models and shows the same essential features as the observed spectra. Analysis of ²⁹Si and ¹⁹⁵Pt NMR linewidths strongly suggests the existence of locally inhomogeneous susceptibility in both materials, and agrees with the widths of the local susceptibility distributions estimated from the susceptibility fits to the disorder-driven NFL models. Disorder-driven mechanisms can also explain the NFL behavior in CePtSi_0.9Ge_0.1; the NMR spectra do not, however, distinguish between the Kondo-disorder and Griffiths phase models. We find that stoichiometric CePtSi and Ge-doped CePtSi_0.9Ge_0.1 show similar degrees of magnetic disorder, although a narrower distribution of local susceptibilities in CePtSi allows Fermi-liquid behavior to appear below 1 K. The residual resistivity reported in CePtSi is relatively large, which indicates a significant level of intrinsic lattice defects and seems to be consistent with the disorder observed in the NMR spectra.

The Np counterpart of the superconducting PuCoGa₅ compound, NpCoGa₅, has been investigated by magnetization, resistivity, specific heat, and ²³⁷Np Mössbauer spectroscopy measurements. Unlike the plutonium compound, NpCoGa₅ does not show any hint of superconductivity down to T=0.4 K but the onset of antiferromagnetic order below T_N=47 K. The magnetization experiments evidence a metamagnetic-like transition (B_c=4.5 T at T=5 K) towards a canted antiferromagnet. The electronic effective mass in NpCoGa₅ is moderately enhanced with a Sommerfeld specific heat coefficient γ=64 mJ mol^-1 K^-2, comparable to that of PuCoGa₅. An ordered Np moment of 0.84 μ_B and the occurrence of a Np³⁺ charge state were inferred from the Mössbauer data. Comparison with NpGa₃ suggests a moderate delocalization of the 5f electrons in NpCoGa₅. Similarities and differences with the isostructural PuCoGa₅ and CeMIn₅ compounds are discussed.

In zero field, Ce₂IrIn₈ obeys Landau’s Fermi-liquid model, with a constant C/T of about 700 mJ/Ce mol K² below 0.7 K and a susceptibility that is constant to ±4% below 4 K. In applied magnetic field, however, Ce₂IrIn₈ shows definite non-Fermi-liquid (nFl) behavior at ∼13 T, with C/T∼ln T between 0.3 and 6 K, χ∼ln T, and ρ=ρ₀+AT¹. At fields of 17 T and higher there is a strong divergent upturn in C/T below 0.7 K that is approximately field independent and the susceptibility becomes again constant (Fermi-liquid like) below 6 K and decreases in magnitude at low temperature compared to χ (13 T). These results imply that a quantum critical point may exist in Ce₂IrIn₈ at ∼13 T. The magnetization at low temperature as a function of field of Ce₂IrIn₈ between 0.1 and 30 T shows no sign of an increase, or jump, near 13 T, but rather a change from M∼H at lower fields to a more saturated behavior above 13 T. Thus, unlike previous field-induced nFl behavior, where the magnetic interactions responsible for the nFl behavior either came (i) at the field, H_metamag, where the magnetization showed a step at a metamagnetic transition (e.g., in UPt₃ or in Sr₃Ru₂O₇), or (ii) at the field where T_Néel in an antiferromagnet was suppressed to T=0 by the field (e.g., in CeCu_6-xAg_x), the present measurements point to a different kind of behavior. Thus the nFl behavior in Ce₂IrIn₈ may be describable as due to quantum criticality at the point in the phase diagram where field induces magnetism. Comparisons to other nFl systems, both field-induced and those which display an anomalous upturn in C/T at low temperatures, are made.

We consider, theoretically and experimentally, the effects of structural disorder, quantum fluctuations, and thermal fluctuations in the magnetic and transport properties of certain ferromagnetic alloys. We study the particular case of UCu₂Si_2-xGe_x. The low temperature resistivity, ρ(T,x), exhibits Fermi liquid behavior as a function of temperature T for all values of x, which can be interpreted as a result of the magnetic scattering of the conduction electrons from the localized U spins. The residual resistivity, ρ(0,x), follows the behavior of a disordered binary alloy. The observed nonmonotonic dependence of the Curie temperature, T_c(x), with x can be explained within a model of localized spins interacting with an electronic bath. Our results clearly show that the Curie temperature of certain alloys can be enhanced due to the interplay between quantum and thermal fluctuations with disorder.

Low temperature specific heat, resistivity, and magnetization for Ce_1-xTh_xRhSb (x=0.2, 0.3 and 0.4), have been investigated. Ce_0.6Th_0.4RhSb and Ce_0.7Th_0.3RhSb show a clear magnetic behavior at 0.35 and 0.15 K, respectively, while Ce_0.8Th_0.2RhSb, with a slight remanent magnetic behavior at ∼0.08 K, shows a non-Fermi-liquid behavior in the specific heat over almost two decades of temperature in the vicinity of x_crit (where T_mag→0 at x_crit), consistent with a quantum critical point scenario. The low temperature specific heat divided by temperature, measured between 0.05 and 8 K, of Ce_0.8Th_0.2RhSb can be fit to either a logarithmic temperature dependence (consistent with various theories for behavior near a quantum critical point) between ∼0.15 and 8 K or to a T^-1+λ temperature dependence (consistent with the Griffiths phase disorder theory) with λ=0.74, but only between 0.05 and 1.3 K. The low temperature magnetic susceptibility, measured down to 1.8 K, of Ce_1-xTh_xRhSb (x=0.2, 0.3, and 0.4) also exhibits a power-law temperature dependence (Griffiths phase model), with an exponent λ (∼0.6) comparable to that found from the specific heat data. Electrical resistivity of Ce_0.8Th_0.2RhSb follows approximately ρ=ρ₀-AT between 0.2 and 2 K with both a very large ρ₀ (1210 μΩ cm) and a gigantic coefficient A (40.7 μΩ cm/K) which is a factor of six larger than the previous record value for a non-Fermi-liquid system found in UCu₄Pd. The possibility that these unusually large values are related to the gap formation seen in pure CeRhSb at low temperatures is discussed. As a further method to resolve whether a quantum critical point or a disorder model best describe this system, the field dependences of the magnetization at 1.8 K and the specific heat down to 0.06 K are compared to predictions for Griffiths phase behavior. There is good agreement between the theory and the magnetization behavior with field, while the specific heat data in field deviate from the theory’s predictions at low temperatures, again displaying a Fermi-liquid behavior below 0.3 K. This reentrance in magnetic field into the Fermi-liquid state below a temperature T^* could be explained by invoking freezing of the spin cluster tunneling due to dissipation effects below a crossover temperature T^*. The physical properties of the Ce_1-xTh_xRhSb system are compared to those found for two non-Fermi-liquid systems: the UCu_5-xPd_x system, where C/T is best fit by log T and some sort of quantum critical scenario (perhaps including spin glass effects) appears to obtain; and the Ce_1-xLa_xRhIn₅ system, where C/T is best fit by T^-1+λ and a Griffiths phase model has been applied.

The large ΔC observed at 17.5 K in URu₂Si₂ is inconsistent with the small, 0.04μ_B moment measured for the antiferromagnetism observed starting (perhaps coincidentally) at the same temperature. We report measurements of this specific-heat transition, thought to be due to some hidden order, in magnetic fields between 24 and 42 T, i.e., through the field-region where three metamagnetic transitions are known to occur at 35.8, 37.3, and 39.4 T. The response of ΔC in single crystal URu₂Si₂ to magnetic field, which includes a change to ΔC being possibly associated with a first-order phase transition for high fields, is analyzed to shed further light on the possible explanations of this unknown ordering process. At fields above 35 T, a new high-field phase comes into being; the connection between this high-field phase revealed by the specific heat and earlier magnetization data is discussed.

We report specific-heat results on a single crystal as well as some polycrystalline samples of Ce_1-xLa_xRhIn₅. Determination of the magnitude of the specific heat γ (≡C/T as T→0) as a function of concentration is made somewhat uncertain by the structure of the specific heat below 3 K. However, within our error bar, this γ (⩽100 mJ/Ce-mol K²)—which differs by approximately a factor of 4 with previous estimates—seems consistent with the effective masses observed in recent de Haas–van Alphen measurements. We find, in addition, that (a) there exists a field-induced transition for x>~0.5 that increases in temperature with increasing applied magnetic field and (b) although single and polycrystalline materials give approximately the same specific heat for x=0.15 and 0.95, the second phase (removable via long-term annealing) in the polycrystalline material plays a role for x=0.5 and 0.8. Furthermore, the low-temperature specific heat shows an upturn in C/T at low temperature for x>~0.5 in both single-crystal and polycrystalline materials, which appears to be intrinsic. The field-induced anomaly, coupled with both the temperature and field dependence of the magnetization data and the temperature dependence of this low-temperature upturn in the zero field C/T (proportional to T^-1+λ) may be evidence for the Griffiths phase non-Fermi-liquid behavior due to the inherent disorder of doped samples.

A low-temperature experiment is reported in which the Mössbauer effect is used to monitor nuclear magnetic resonance in the I=1/2 ground state of ⁵⁷Fe. Contrary to an earlier attempt at this experiment the resonance is single line. Despite the enriched 30 at. % ⁵⁷Fe concentration of the iron foil specimen, the NMR line broadening is predominantly inhomogeneous in nature.

The specific heat as a function of temperature between 1.4 and 10 K for fields between 28 and 45 T reveals a small anomaly in the C/T data that is first visible at 32 T. The anomaly grows in size monotonically up to our highest field of measurement (45 T); the temperature of the maximum of the specific heat anomaly grows as well with increasing field at approximately 0.21 K/T, up to a value of T_peak=4.1 K at 45 T. Thermodynamic considerations indicate that the field-induced transition is first order. The data are compared to the weak metamagnetic transition in the magnetization observed around 42 T by Takeuchi et al.

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 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.

The low-temperature specific heat, in magnetic fields to 32 T, and the magnetic susceptibility are reported on single crystals of the new heavy-fermion superconductors CeMIn₅, M=Ir and Co, as well as the new heavy-fermion antiferromagnet CeRhIn₅. The absence of a pronounced field dependence to the specific heat of the Ir and Co systems suggests that the large Sommerfeld coefficients of these compounds are due to correspondingly large effective electron masses. In addition, the indicated non-Fermi-liquid behavior previously suggested from the temperature dependence of the resistivity of CeIrIn₅ has been confirmed in our measurements of the susceptibility and specific heat for this compound, as well as in the susceptibility and specific heat of CeCoIn₅. The existence of superconductivity in these systems that appears, based on their non-Fermi-liquid behavior, to develop near a quantum critical point is further support for this superconductivity being of unconventional nature.