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We report the results of optical studies of new heavy fermion compounds YbFe(4)Sb(12) and CeRu(4)Sb(12). We show that these compounds, as well as several other heavy fermion materials with a nonmagnetic ground state, obey a universal scaling relationship between the quasiparticle effective mass m(*) and the magnitude of the energy gap Delta in the excitation spectrum. This result is in accord with the picture of hybridization of localized f-electron and free carrier states.

The trigonal pyramidal complex [Mn4O3Cl(O2CCH3)3(dbm)3], where dbm- is the monoanion of dibenzoylmethane, functions as a single-molecule magnet. High-field EPR data are presented for an oriented microcrystalline sample to characterize the electronic structure of the MnIVMnIII 3 complex. These data show that the complex has a S = 9/2 ground state, experiencing axial zero-field splitting (DŜ z 2) with D = −0.53 cm-1 and a quartic zero-field splitting (B 4 0Ô4 0)with B 4 0 = −7.3 × 10-5 cm-1. Magnetization versus external magnetic field data were collected for an oriented single crystal in the 0.426−2.21 K range. At temperatures below 0.90 K hysteresis is seen. Steps are seen on each hysteresis loop. This is clear evidence that each MnIVMnIII 3 complex functions as a single-molecule magnet that is magnetizable. Furthermore, the steps on the hysteresis loops are due to resonant magnetization quantum mechanical tunneling. In response to an external field each molecule reverses its direction of magnetization not only by being thermally activated over a potential-energy barrier, but by the magnetization tunneling through the barrier. Additional evidence for resonant magnetization tunneling was found in the change in the temperature at which the out-of-phase ac magnetic susceptibility is observed as a function of an external dc field. The results of magnetization relaxation experiments carried out in the 0.394−0.700 K range are presented. These data are combined with the ac susceptibility data taken at higher temperatures to give an Arrhenius plot of the logarithm of the magnetization relaxation rate versus inverse absolute temperature. The temperature-dependent part of this plot gives an activation barrier of 11.8 K. Below 0.6 K the relaxation rate is independent of temperature with a rate of 3.2 × 10-2 s-1. This S = 9/2 single-molecule magnet exhibits a tunneling of its direction of magnetization at a rate of 3.2 × 10-2 s-1 in the 0.394−0.600 K range. Thus, resonant magnetization tunneling is seen for a half-integer-spin (S = 9/2) ground-state magnet in the absence of an external magnetic field. The transverse component of the small magnetic field from the nuclear spins is probably the origin of this tunneling.

(ReceivedWe report ferromagnetism at over 900 K in Cr-GaN and Cr-AlN thin films. The magnetic properties vary as a function of Cr concentration with 60%, and 20%, of the Cr being magnetically active at 3% doping in GaN, and 7% in AlN, respectively. In the GaN sample with the highest magnetically active Cr (60%), channeling Rutherford Backscattering indicates that over 70% of Cr impurities are located on substitutional sites. These results give indisputable evidence that substitutional Cr defects are involved in the magnetic behavior. While Cr-AlN is highly resistive, Cr-GaN exhibits properties characteristic of hopping conduction including T 1/2 resistivity dependence and small Hall mobility (0.06 cm 2 /V . s). A large negative magnetoresistance is attributed to the influence of the magnetic field on the quantum interference between the many paths linking two hopping sites. The results strongly suggest that ferromagnetism in Cr-GaN and Cr-AlN can be attributed to the double exchange mechanism as a result of hopping between near-midgap substitutional Cr impurity bands. Cr-GaN, 5 and Cr-AlN. 6 (Ga,Mn)N films grown by molecular beam epitaxy were reported to be ferromagnetic above room temperature, with a Curie temperature, T c , of 940 K. 7 Another study of the same system synthesized using solid-state diffusion reported a T c in the range of 220-370 K, 3 while bulk Cr-doped GaN fabricated using the sodium flux method was reported to have a T c of 280 K. 5 The search for the physical mechanism responsible for the observed ferromagnetic properties and the question of the applicability of the classical magnetic models have become topics of intense interest. This letter reports the observation of ferromagnetism in Cr-GaN and Cr-AlN above 900 K, and describes the structural, electrical and magnetic properties of the materials.The Cr-GaN and Cr-AlN films were grown on 6H-SiC (0001) and sapphire (0001) substrates in a reactive molecular beam epitaxy system. Structural properties were characterized using X-ray diffraction ( Magnetic fields were applied parallel to the film plane during susceptibility measurements and perpendicular to the film during magnetoresistance (MR) measurements. The diamagnetic background contributions originating from the substrate and the sample holder were subtracted

Several quantum paramagnets exhibit magnetic field-induced quantum phase transitions to an antiferromagnetic state that exists for Hc1 ≤ H ≤ Hc2. For some of these compounds, there is a significant asymmetry between the low-and high-field transitions. We present specific heat and thermal conductivity measurements in NiCl2-4SC(NH2)2, together with calculations which show that the asymmetry is caused by a strong mass renormalization due to quantum fluctuations for H ≤ Hc1 that are absent for H ≥ Hc2. We argue that the enigmatic lack of asymmetry in thermal conductivity is due to a concomitant renormalization of the impurity scattering. PACS numbers: 75.10.Jm, 75.40.Cx The correspondence between a spin system and a gas of bosons has been very fruitful for describing field-induced ordered phases in a large class of quantum paramagnets [1][2][3][4][5]. In this analogy, a magnetic field H plays the role of the chemical potential, which, upon reaching a critical value H c1 , induces a T = 0 Bose-Einstein condensation (BEC), provided that the number of bosons is conserved, the kinetic energy is dominant, and the spatial dimension d > 1. Such a BEC state corresponds to a canted XY magnetic ordering of the spins.At the BEC quantum critical point (QCP), the low-energy bosonic excitations have a quadratic dispersion ω = k 2 /2m * , where m * is the effective mass. This mass is renormalized by quantum fluctuations in the paramagnetic phase H ≤ H c1 . In magnets with H c1 H c2 the renormalization can be expected to be very strong because of the proximity to the magnetic instability. The transition at H c1 should be contrasted with the second BEC-QCP that takes place at the saturation field H c2 [6]. Since the field induced magnetization is a conserved quantity, there are no quantum fluctuations and no mass renormalization for the fully polarized phase above H c2 , i.e., the bare mass m can be obtained from the single-particle excitation spectrum at H ≥ H c2 . Thus, quantum paramagnets are ideal for studying mass renormalization effects because the effective and the bare bosonic masses can be obtained from two different QCP's that occur in the same material.Here we present theoretical and experimental evidence for a strong mass renormalization effect, m/m * 3, in NiCl 2 -4SC(NH 2 ) 2 [referred to as DTN]. We will show that the large asymmetry between the peaks in the low-temperature specific heat, C v (H), in the vicinity of H c1 and H c2 is closely described by analytical and Quantum Monte Carlo (QMC) calculations. The mass renormalization also explains similar asymmetries observed in other properties of DTN, such as magnetization [7], electron spin resonance [8], sound velocity [9,10], and magnetostriction [11]. In a remarkable contrast to these properties, peaks in the low-temperature thermal conductivity, κ, near H c1 and H c2 do not show any substantial asymmetry. We provide an explanation to this dichotomy by demonstrating that the leading boson-impurity scattering amplitude is also renormalized by quantum fluctuation...

The non-Fermi-liquid (NFL) behavior observed in the low temperature specific heat C(T ) and magnetic susceptibility χ(T ) of f-electron systems is analyzed within the context of a recently developed theory based on Griffiths singularities. Measurements of C(T ) and χ(T ) in the systems Th1−xUxPd2Al3, Y1−xUxPd3, and UCu5−xMx (M = Pd, Pt) are found to be consistent with C(T )/T ∝ χ(T ) ∝ T −1+λ predicted by this model with λ < 1 in the NFL regime. These results suggest that the NFL properties observed in a wide variety of f-electron systems can be described within the context of a common physical picture. PACS: 71.27+a, 75.20.Hr, 71.10.Hf Transport, thermal, and magnetic measurements on a number of chemically substituted rare earth and actinide compounds have revealed low temperature physical properties that show striking departures from the predictions of Fermi-liquid theory [1]. Several theoretical models have been developed to account for the non-Fermi-liquid (NFL) behavior observed in f-electron materials. These models include a multichannel Kondo effect of magnetic or electric origin [2-4], fluctuations of an order parameter in the vicinity of a second order phase transition at T = 0 K [5-7], a disordered distribution of Kondo temperatures [8,9], and an electron polaron model for heavy fermion systems [10]. However, none of these models has been able to account for all of the NFL characteristics observed in the wide variety of systems that belong to this new class of strongly correlated f-electron materials. Three of us (A. H. C. N., G. E. C., and B. A. J.) have recently proposed a model where NFL behavior is associated with the proximity to a quantum critical point and the formation of magnetic clusters in the paramagnetic phase due to the competition between the Kondo effect and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in the presence of magnetic anisotropy and disorder inherent in alloyed materials [11]. This model predicts that various physical properties diverge with decreasing temperature as weak power laws of temperature and that this behavior persists over appreciable ranges of substituent concentration, similar to what has been observed in a number of f-electron materials.

In this paper, we present systematic measurements of the temperature and magnetic field dependences of the thermodynamic and transport properties of the Yb-based heavy fermion YbPtBi for temperatures down to 0.02 K with magnetic fields up to 140 kOe to address the possible existence of a field-tuned quantum critical point. Measurements of magnetic field and temperature dependent resistivity, specific heat, thermal expansion, Hall effect, and thermoelectric power indicate that the AFM order can be suppressed by applied magnetic field of H c ∼ 4 kOe. In the H − T phase diagram of YbPtBi, three regimes of its low temperature states emerges: (I) AFM state, characterized by spin density wave (SDW) like feature, which can be suppressed to T = 0 by the relatively small magnetic field of H c ∼ 4 kOe, (II) field induced anomalous state in which the electrical resistivity follows ∆ρ(T ) ∝ T 1.5 between H c and ∼ 8 kOe, and (III) Fermi liquid (FL) state in which ∆ρ(T ) ∝ T 2 for H ≥ 8 kOe. Regions I and II are separated at T = 0 by what appears to be a quantum critical point. Whereas region III appears to be a FL associated with the hybridized 4f states of Yb, region II may be a manifestation of a spin liquid state.

The low temperature properties of polycrystalline samples of the filled skutterudites and , as well as the unfilled skutterudites and , have been investigated by means of electrical resistivity, specific heat and magnetic susceptibility measurements. The resistivity of exhibits a rather abrupt drop-off with decreasing temperature near 100 K; this drop-off temperature increases with increasing applied hydrostatic pressure, which is reminiscent of the onset of coherence in so-called Kondo lattice materials. The compounds and exhibit values of the electronic specific heat coefficient and Pauli susceptibility at low temperature which are enhanced over those of the lanthanum-filled and unfilled skutterudites. These quantities yield a Wilson ratio of order unity, which indicates that they both correspond to the properties of itinerant electrons. These transport, magnetic and thermodynamic properties suggest a moderately heavy fermion ground state in and .