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We have studied the terahertz response of the rare-earth orthoferrite $\text{Er}_{x}\mathrm{Y}_{1-x}\text{FeO}_{3}$ as a function of magnetic field, temperature, and yttrium composition ( $x$ ). Through theoretical modelling based on an extended Dicke Hamiltonian, we provide new insight into the role of cooperative, “superradiant” interactions between Er spins and Fe magnons in the low-temperature phase transition that this magnetic system exhibits.

Triangular lattice of rare-earth ions with interacting effective spin-$1/2$ local moments is an ideal platform to explore the physics of quantum spin liquids (QSLs) in the presence of strong spin-orbit coupling, crystal electric fields, and geometrical frustration. The Yb delafossites, NaYbCh$_2$ (Ch=O, S, Se) with Yb ions forming a perfect triangular lattice, have been suggested to be candidates for QSLs. Previous thermodynamics, nuclear magnetic resonance, and muon spin rotation measurements on NaYbCh$_2$ have supported the suggestion of the QSL ground states. The key signature of a QSL, the spin excitation continuum, arising from the spin quantum number fractionalization, has not been observed. Here we perform both elastic and inelastic neutron scattering measurements as well as detailed thermodynamic measurements on high-quality single-crystalline NaYbSe$_2$ samples to confirm the absence of long-range magnetic order down to 40 mK, and further reveal a clear signature of magnetic excitation continuum extending from 0.1 to 2.5 meV. The comparison between the structure of the magnetic excitation spectra and the theoretical expectation from the spinon continuum suggests that the ground state of NaYbSe$_2$ is a QSL with a spinon Fermi surface., Comment: 18 pages, 6 figures

We report a chemical substitution-induced ferromagnetic quantum critical point in polycrystalline Ni$_{1-x}$Rh$_x$ alloys. Through magnetization and muon spin relaxation measurements, we show that the ferromagnetic ordering temperature is suppressed continuously to zero at $x_{crit} = 0.375$ while the magnetic volume fraction remains 100% up to $x_{crit}$, pointing to a second order transition. Non-Fermi liquid behavior is observed close to $x_{crit}$, where the electronic specific heat $C_{el}/T$ diverges logarithmically, while immediately above $x_{crit}$ the volume thermal expansion coefficient $\alpha_{V}/T$ and the Gr\"uneisen ratio $\Gamma = \alpha_{V}/C_{el}$ both diverge logarithmically in the low temperature limit, further indication of a ferromagnetic quantum critical point in Ni$_{1-x}$Rh$_x$., Comment: 15 pages, 4 figures

Ferromagnetic Kondo lattice compounds are far less common than their antiferromagnetic analogs. In this work, we report the discovery of a new ferromagnetic Kondo lattice compound, YbIr3Ge7. Like almost all ferromagnetic Kondo lattice systems, YbIr3Ge7 shows magnetic order with moments aligned orthogonal to the crystal electric field (CEF) easy axis. YbIr3Ge7 is unique in that it is the only member of this class of compounds that crystallizes in a rhombohedral structure with a trigonal point symmetry of the magnetic site, and it lacks broken inversion symmetry at the local moment site. AC magnetic susceptibility, magnetization, and specific heat measurements show that YbIr3Ge7 has a Kondo temperature TK = 14 K and a Curie temperature TC = 2.4 K. Ferromagnetic order occurs along the crystallographic [100] hard CEF axis despite the large CEF anisotropy of the ground state Kramers doublet with a saturation moment along [001] almost four times larger than the one along [100]. This implies that a mechanism which considers the anisotropy in the exchange interaction to explain the hard axis ordering is unlikely. On the other hand, the broad second-order phase transition at TC favors a fluctuation-induced mechanism.

Sharp magnetization switching and large magnetoresistance (MR) were previously discovered in single crystals of 2H-${\mathrm{Fe}}_{x}{\mathrm{TaS}}_{2}$ and attributed to the Fe superstructure and its defects. We report similar sharp switching and large MR in 1T-${\mathrm{Fe}}_{x}{\mathrm{TiS}}_{2}$ ($0.086\phantom{\rule{0.28em}{0ex}}\ensuremath{\le}\phantom{\rule{0.28em}{0ex}}x\phantom{\rule{0.28em}{0ex}}\ensuremath{\le}0.703$) while providing a side-by-side comparison of the only two such ferromagnetic transition-metal dichalcogenides. The switching field ${H}_{s}$ and MR values are similar in both 1T-${\mathrm{Fe}}_{x}{\mathrm{TiS}}_{2}$ and 2H-${\mathrm{Fe}}_{x}{\mathrm{TaS}}_{2}$, with a larger than expected bowtie $\ensuremath{\rho}(H)$ and a sharper hysteresis loop for $H\ensuremath{\parallel}c$ in the former. The Curie and Weiss temperatures remain roughly constant below $x\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}1/3$ in the $T=\text{Ti}$ single crystals before increasing monotonically for higher $x$, while ${H}_{s}$ and MR reach maxima where defects in the superstructure exist, or a minimum near superstructure compositions, and they remain constant above $x\ensuremath{\sim}0.4$. Despite previous reports, electron diffraction shows only the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ superstructure in 1T-${\mathrm{Fe}}_{x}{\mathrm{TiS}}_{2}$. Glassy behavior is shown to coexist within the ferromagnetic state in 1T-${\mathrm{Fe}}_{x}{\mathrm{TiS}}_{2}$ for compositions between 0.1 and 0.703. A simple theoretical model considering first-, second- and third-neighbor interactions yields a phase diagram that accounts for both spin glass behavior and different superstructures.

A quantum spin liquid (QSL) is a state of matter where unpaired electrons' spins in a solid are quantum entangled, but do not show magnetic order in the zero-temperature limit. Because such a state may be important to the microscopic origin of high-transition temperature superconductivity and useful for quantum computation, the experimental realization of QSL is a long-sought goal in condensed matter physics. Although neutron scattering experiments on the two-dimensional (2D) spin-1/2 kagome-lattice ZnCu3(OD)6Cl2 and effective spin-1/2 triangular lattice YbMgGaO4 have found evidence for a continuum of magnetic excitations, the hallmark of a QSL carrying 'fractionalized quantum excitations', at very low temperature, magnetic and nonmagnetic site chemical disorder complicates the interpretation of the data. Recently, the three-dimensional (3D) Ce3+ pyrochlore lattice Ce2Sn2O7 has been suggested as a clean, effective spin-1/2 QSL candidate, but there is no evidence of a spin excitation continuum. Here we use thermodynamic, muon spin relaxation ({\mu} SR), and neutron scattering experiments on single crystals of Ce2Zr2O7, a compound isostructural to Ce2Sn2O7, to demonstrate the absence of magnetic ordering and the presence of a spin excitation continuum at 35 mK, consistent with the expectation of a QSL. Since our neutron diffraction and diffuse scattering measurements on Ce2Zr2O7 reveal no evidence of oxygen deficiency and magnetic/nonmagnetic ion disorder as seen in other pyrochlores, Ce2Zr2O7 may be the first example of a 3D QSL material with minimum magnetic and nonmagnetic chemical disorder.

${\mathrm{CeIr}}_{3}{\mathrm{Ge}}_{7}$ is an antiferromagnetic metal with a remarkably low ordering temperature ${T}_{\mathrm{N}}=0.63\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, while most Ce-based magnets order between 2 and 15 K. Thermodynamic and transport properties as a function of magnetic field or pressure do not show signatures of Kondo correlations, interaction competition, or frustration, as had been observed in a few antiferromagnets with comparably low or lower ${T}_{\mathrm{N}}$. The averaged Weiss temperature measured below 10 K is comparable to ${T}_{\mathrm{N}}$, suggesting that the Ruderman-Kittel-Kasuya-Yosida exchange coupling is very weak in this material. The unusually low ${T}_{\mathrm{N}}$ in ${\mathrm{CeIr}}_{3}{\mathrm{Ge}}_{7}$ can therefore be attributed to the large Ce-Ce bond length of about 5.7 \AA{}, which is about 1.5 \AA{} larger than in the most Ce-based intermetallic systems.

Kondo-based semimetals and semiconductors are of extensive current interest as a viable platform for strongly correlated states. It is thus important to understand the routes towards such dilute-carrier correlated states. One established pathway is through Kondo effect in metallic non-magnetic analogues. Here we advance a new mechanism, through which Kondo-based semimetals develop out of conduction electrons with a low carrier-density in the presence of an even number of rare-earth sites. We demonstrate this effect by studying the Kondo material Yb3Ir4Ge13 along with its closed-f-shell counterpart, Lu3Ir4Ge13. Through magnetotransport, optical conductivity and thermodynamic measurements, we establish that the correlated semimetallic state of Yb3Ir4Ge13 below its Kondo temperature originates from the Kondo effect of a low carrier conduction-electron background. In addition, it displays fragile magnetism at very low temperatures, which, in turn, can be tuned to a non Fermi liquid regime through Lu-for-Yb substitution. These findings are connected with recent theoretical studies in simplified models. Our results open an entirely new venue to explore the strong correlation physics in a semimetallic environment.

The ferromagnetic (FM) phase transition of the itinerant electron-system Sr1−x Ca x RuO3 can be tuned by chemical composition resulting in a quantum critical point (QCP) at the critical concentration x c ≈ 0.7. Applying epitaxial pressure at constant x leads to a reduction of the Curie temperature T C which is found to be proportional to the shrinkage of the unit-cell volume V uc , shifting x c to higher values for tensile strained films. Surprisingly, the tetragonal distortion seems to play here only a minor role. With increasing x the critical scaling of the order parameter shows unusual behavior. The magnetic critical exponents β, γ, and δ change systematically from typical mean-field values at x = 0 with increasing x towards β = 1, γ = 0.9 and δ = 1.6 at x = 0.7. The results are discussed with respect to a crossover from mean-field-like behavior at x = 0 to a line of fixed points that might emerge in the strong-disorder limit as the system approaches the QCP at or near x c . Magnetic inhomogeneities are indeed suggested by a non-vanishing magnetic moment at x c and the evidence of a Griffiths phase as well as glass-like behavior close to x c . Although spin fluctuations certainly play an important role around x c as proposed previously, our highly accurate data of the magnetization M(T,B) and specific heat C(T,B) for x = 0.7 suggest dynamic scaling with an unusual dynamic exponent z = 1.8, incompatible with standard spin-fluctuation theories at a ferromagnetic QCP.

In this review, an overview of itinerant magnets without magnetic elements is presented, beginning with a comparison of the local and itinerant moment pictures, the two extremes of magnetism. Then, the theoretical developments leading up to the self-consistent renormalization theory of spin fluctuations will be discussed, followed by an introduction to quantum criticality and the experimental signatures associated with systems near a quantum critical point. Three itinerant magnets without magnetic elements, ZrZn2, Sc3.1In, and TiAu are the focus of this review, as their empty d shells set them apart in their purely itinerant character, while several enhanced Pauli paramagnets and intermediate moment magnets are also discussed to put the overall comparison into perspective.

In the heavy-fermion metal CePdAl long-range antiferromagnetic order coexists with geometric frustration of one third of the Ce moments. At low temperatures the Kondo effect tends to screen the frustrated moments. We use magnetic fields $B$ to suppress the Kondo screening and study the magnetic phase diagram and the evolution of the entropy with $B$ employing thermodynamic probes. We estimate the frustration by introducing a definition of the frustration parameter based on the enhanced entropy, a fundamental feature of frustrated systems. In the field range where the Kondo screening is suppressed the liberated moments tend to maximize the magnetic entropy and strongly enhance the frustration. Based on our experiments, this field range may be a promising candidate to search for a quantum spin liquid., Comment: 5+2 pages with 4 figures

CePdAl crystallizes in the hexagonal ZrNiAl-type structure (space group ), where the magnetic Ce3+ ions form a network of equilateral triangles in the ab plane similar to the kagome lattice. It is a well known antiferromagnetic heavy-fermion system with Neel temperature TN = 2.7 K. Magnetization and specific-heat measurements performed on CePd1 − xNixAl polycrystals show that the antiferromagnetic order in the system is suppressed by Ni substitution. TN is reduced to zero temperature at a Ni concentration x = 0.144. At high temperatures the magnetization data follow a Curie–Weiss law. The Weiss temperature is negative with increasing with x possibly due to the increasing Kondo effect. Indeed, the Kondo temperature increases with x as documented by an analysis of the specific heat and magnetic entropy of CePd1 − xNixAl for the samples with x = 0, 0.062, 0.097, and 0.144.

This paper presents a novel scheme for measuring the axial and radial displacements of a miniature magnetically levitated rotor based on linear Hall-effect sensors. The measurement system consists of six single-axis Hall-effect sensors attached to the stator with a ring shaped permanent magnet (PM) mounted on the rotor as the magnetic flux source. The system is configured such that the directions of the magnetic flux into the sensors for each axis are monitored. Consequently, the errors caused by uneven magnetization and the temperature dependence of the PM are minimized. The experimental results demonstrate measurement system satisfactory performance. Since only single-axis Hall sensors are used in the measurement system, this scheme provides a low-cost compact size solution for three-dimensional displacement measurement of miniature magnetically levitated rotors.

In miniature magnetically levitated rotating machines, permanent magnet bearings are generally used for rotor levitation in the radial directions, while and electromagnetic actuators are used for rotor stabilization in the axial direction. In this configuration, radial rotor oscillations are unavoidable because no controllable force is applicable in these directions. Consequently, rotor damping relies entirely on the media in the air gap. This paper presents a novel electromagnetic actuator for miniature magnetically levitated rotating machines. The actuator stator core consists of four poles for flux passage. The currents on these poles are independently controlled so that axial and radial forces are produced. The axial force is used for rotor levitation control, while the radial force is used to assist in radial oscillation suppression. Theoretical analysis and experimental verification for the radial force production are provided.

We present measurements of the magnetization $M$, specific heat $C$, resistivity $\ensuremath{\rho}$, and magnetoresistance MR of single-crystalline hexagonal CeAuSn for temperature down to $T=1.6 \mathrm{K}$ and in magnetic field up to $B=12 \mathrm{T}$. Antiferromagnetic ordering at ${T}_{\mathrm{N}}=4.4 \mathrm{K}$ is observed as previously found for polycrystalline samples. A strong magnetic easy-plane anisotropy of $M$ for $B$ perpendicular and parallel to the $c$ direction is found with ${M}_{\ensuremath{\perp}}/{M}_{||}\ensuremath{\approx}15$ in $B=0.1 \mathrm{T}$ around ${T}_{\mathrm{N}}$, which is attributed to crystal-electric-field anisotropy. The analysis of the magnetic susceptibility indicates ferromagnetic correlations above ${T}_{\mathrm{N}}$. Measurements of $M(T)$ under hydrostatic pressure $P$ show that ${T}_{\mathrm{N}}(P)$ increases linearly with $P$ at a small rate of 0.035 K/kbar up to 4 kbar and gradually saturates approaching $P=16\phantom{\rule{0.28em}{0ex}}\mathrm{kbar}$. Zero-field $\ensuremath{\Delta}C/T$, the phonon contribution to $C$ being subtracted, is proportional to ${T}^{2}$ below ${T}_{\mathrm{N}}$ indicating a gapless spin-wave spectrum. It is found that all $\ensuremath{\Delta}C(T,B)/T$ curves for $B=0--9 \mathrm{T}$ cross at the same temperature, providing an example of a particularly well defined isosbestic point in a very narrow region around ${T}_{\mathrm{iso}}=6.6 \mathrm{K}$. Finally, $\ensuremath{\rho}(T)$ and MR experiments with current perpendicular and parallel to $B$ allow us to separate orbital effects from the Zeeman splitting.

The dynamics of continuous phase transitions is governed by the dynamic scaling exponent relating the correlation length and correlation time. For transitions at finite temperature, thermodynamic critical properties are independent of the dynamic scaling exponent. In contrast, at quantum phase transitions where the transition temperature becomes zero, static and dynamic properties are inherently entangled by virtue of the uncertainty principle. Consequently, thermodynamic scaling equations explicitly contain the dynamic exponent. Here we report on thermodynamic measurements (as a function of temperature and magnetic field) for the itinerant ferromagnet Sr1-xCaxRuO3 where the transition temperature becomes zero for x=0.7. We find dynamic scaling of the magnetization and specific heat with highly unusual quantum critical dynamics. We observe a small dynamic scaling exponent of 1.76 strongly deviating from current models of ferromagnetic quantum criticality and likely being governed by strong disorder in conjunction with strong electron-electron coupling.

The critical behavior of ${\mathrm{Sr}}_{1\ensuremath{-}x}$${\mathrm{Ca}}_{x}$${\mathrm{RuO}}_{3}$ was investigated by a scaling analysis based on the Arrott-Noakes equation of state. The critical exponents \ensuremath{\beta}, \ensuremath{\gamma}, and \ensuremath{\delta} of the magnetic critical behavior were extracted for samples with 0 $\ensuremath{\le}x\ensuremath{\le}0.6$ . The ferromagnetic system exhibits a smooth suppression of the Curie temperature ${T}_{\mathrm{C}}$ to zero at a critical concentration ${x}_{c}$ \ensuremath{\approx} 0.7. The ordered magnetic moment decreases simultaneously as expected for itinerant ferromagnets, however, does not vanish completely at ${x}_{\mathrm{c}}$, indicating small magnetic clusters or inhomogeneities. For $x$ = 0, mean-field like exponents are observed. With increasing $x$, the critical exponents \ensuremath{\beta}, \ensuremath{\gamma}, and \ensuremath{\delta} vary nearly linearly from \ensuremath{\beta} \ensuremath{\approx} 0.5, \ensuremath{\gamma} \ensuremath{\approx} 1, and \ensuremath{\delta} \ensuremath{\approx} 3 for $x$ = 0 to \ensuremath{\beta} \ensuremath{\approx} 1, \ensuremath{\gamma} \ensuremath{\approx} 0.9, and \ensuremath{\delta} \ensuremath{\approx} 1.6 for $x$ \ensuremath{\approx} 0.6. The Widom scaling relation is always met for $x\ensuremath{\le}0.6.$ Despite the systematic evolution of the critical exponents as a function of $x$, the exponents cannot be described by any of the universality classes known for classical standard models. The particular trend of the effective critical exponents may be possibly explained by a strong-disorder line of fixed points; the vicinity to ${x}_{c}$ suggests that this behavior is caused by the vicinity to a quantum phase transition.

Quantum phase transitions have captured the interest of a large community in condensed-matter and atom physics research. The common feature of these very different material classes lies in the fact that the competition between low-energy scales can be tuned by a nonthermal parameter, such as pressure, magnetic or electric field, and chemical composition for the condensed-matter systems. In heavy-fermion materials, the strong exchange J between f-electrons and conduction electrons can lead to quenching of the f-electron-derived (nearly) localized magnetic moments via the Kondo effect or, if J becomes weaker, to long-range magnetic order via the Ruderman-Kittel-Kasuya-Yosida interaction mediated by the conduction electrons. In addition it has been suggested that magnetic order can be suppressed by quantum fluctuations which may be enhanced by geometric frustration. Here we report on the observation of a quantum phase transition in a partially frustrated antiferromagnetic metallic system. In antiferromagnetic CePdAl the magnetic Ce ions form a network of equilateral triangles in the (001) plane, similar to the kagom\'e lattice, with one third of the Ce moments not participating in long-range order. The N\'eel temperature T_N = 2.7 K can be driven to zero upon replacing 14.4% of Pd by Ni. Here the specific heat C exhibits a C/T ~ - log T dependence. Within the Hertz-Millis-Moriya model of quantum criticality, this behavior can be attributed to two-dimensional critical antiferromagnetic fluctuations arising from the decoupling of three-dimensional magnetic order by frustration. The intermediate planes of frustrated moments are a possible candidate for a two-dimensional spin-liquid. The simultaneous presence of magnetic order, geometric frustration, and Kondo effect in this system might thus entail a new route to quantum criticality., Comment: 6 pages, 5 figures

We report the magnetic field dependent dc magnetization and the pressure-dependent (pmax ~ 16 kbar) ac susceptibilities Xp(T) on both powder and bulk multiferroic BiMnO3 samples, synthesized in different batches under high pressure. A clear ferromagnetic (FM) transition is observed at TC ~ 100 K, and increases with magnetic field. The magnetic hysteresis behavior is similar to that of a soft ferromagnet. Ac susceptibility data indicate that both the FM peak and its temperature (TC) decrease simultaneously with increasing pressure. Interestingly, above a certain pressure (9 ~ 11 kbar), another peak appears at Tp ~ 93 K, which also decreases with increasing pressure, with both these peaks persisting over some intermediate pressure range (9 ~ 13 kbar). The FM peak disappears with further application of pressure; however, the second peak survives until present pressure limit (pmax ~ 16 kbar). These features are considered to originate from the complex interplay of the magnetic and orbital structure of BiMnO3 being affected by pressure., Comment: 4 pages,4 figures, publised in Physical Review B 78, 092404/2008

The magnetic phase variations under hydrostatic pressure on multiferroic BiMnO3 have been examined by the dc magnetization [Mg(T)], magnetic hysteresis [Ueff(H)], and ac susceptibility [X'g(T)]. Three magnetic transitions, manifested as kinks I, II, and III on the Mg(T)], curves, were identified at 8.7 and 9.4 kbar. With increasing pressure, transition temperatures of kink I and kink II TkI and TkII tend to decrease, but the temperature of kink III TkIII showed more complex variation. Under increasing magnetic field, TkI and TkII increase; however, TkIII decreases. Combining [Mg(T)] curves with Ueff(H) and X'g(T), more detailed properties of these three kinks would be shown as follows. Kink I is a long-range soft ferromagnetic transition which occurs at TkI 100 K under ambient pressure but is suppressed completely at 11.9 kbar. Kink II emerges at 8.7 kbar along with TkII 93 K which is also long-range soft ferromagnetic but canted in nature. Kink III, a canted antiferromagnetic transition, appears at TkIII 72.5 K along with kink II also at 8.7 kbar. The proposed phase diagrams at ambient pressure, 9.4 and 11.9 kbar show the different magnetic features of BiMnO3. These findings are believed to result from the variations in crystal structure influenced by the external pressure. These results also indicate the common complicated, 6 paages, 5 gifures, published in Physical Review B 80, 184426/2009

In miniature magnetically levitated rotating machines, permanent magnet bearings are generally used for rotor levitation in the radial directions and electro-magnetic actuators are used for rotor stabilization in the axial direction. In this configuration rotor radial oscillations are unavoidable because no controllable force is applicable in these directions. Consequently, rotor damping relies entirely on the media in the air-gap. This paper presents a novel electro-magnetic actuator for miniature magnetically levitated rotating machine. The actuator stator core consists of four poles for flux passage. The currents on these poles are independently controlled so that axial and radial forces are produced. The axial force is used for rotor levitation control, while the radial force is used to assist in radial oscillation suppression. Theoretical analysis and experimental verifications for the radial force production are provided.

In miniature magnetically levitated rotating machines, permanent magnet bearings are generally used for rotor levitation in the radial directions, while and electromagnetic actuators are used for stabilization in the axial direction. In this configuration rotor radial oscillations are inevitable because no controllable force is applicable in these directions. This paper presents the implementation and control scheme of a electro-magnetic actuator for these machines. The actuator can produce axial as well as radial forces on the rotor. The axial force is used for rotor axial direction control, and the radial force is used to assist the suppression of rotor resonance related oscillations. Theoretical analysis and experimental investigations of the actuator control are performed, and the results are provided in the paper.

This paper presented a scheme to measure axial and radial displacement of a magnetically levitated rotor based on linear Hall-effect sensors. The measurement system consists of six single-axis Hall sensors attached on the stator, and a ring permanent magnet mounted on the rotor as the magnetic flux source. The system is configured such that the angles of the magnetic flux into the sensors are monitored. Consequently, the errors caused by non-uniform magnetization and temperature dependence are minimized. The experimental results demonstrated a satisfactory performance of the measurement system. Since single-axis Hall sensors are used, this scheme provides a low-cost and compact in size solution for three- dimensional displacement measurement for levitated rotors.

To resolve the discrepancies of the superconducting order parameter in quasi-two-dimensional NbSe2, comprehensive specific-heat measurements have been carried out. By analyzing both the zero-field and mixed-state data with magnetic fields perpendicular (H perpendicular to c) to and parallel (H parallel to c) to the c axis of the crystal and using the two-gap model, we conclude that (1) more than one energy scale of the order parameter is required for superconducting NbSe2 due to the thermodynamic consistency; (2) Delta(L)=1.26 meV and Delta(S)=0.73 meV are obtained; (3) N-S(0)/N(0)=11%-20%; (4) the observation of the kink in gamma(H) curve suggests that the two-gap scenario is more favorable than the anisotropic s-wave model to describe the gap structure of NbSe2; and (5) Delta(S) is more isotropic and has a three-dimensional-like feature and is located either on the Se or the bonding Nb Fermi sheets.

Low temperature magnetic and transport properties of the sol-gel derived Cd doped ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}{\mathrm{O}}_{3}(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.20)$ manganite system show ferromagnetism and cluster spin-glass behavior. Metal-insulator transition (MIT) is exhibited only by the samples with $x\ensuremath{\leqslant}0.1$; and for higher concentrations ($x=0.15$ and 0.20), samples are semiconductors. The MIT shifts to the lower temperature regime with increasing Cd content, indicating an increase of disorder in the system. Structural analysis shows local strain-induced small lattice distortion of the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra with Cd doping. With increasing Cd content, the system undergoes interesting paramagnetic to ferromagnetic (with $xl0.10$) as well as paramagnetic to cluster spin-glass (for $x\ensuremath{\geqslant}0.10$) transitions. ac susceptibility $({\ensuremath{\chi}}_{\mathrm{ac}})$ and magnetic relaxation measurements confirm this cluster spin-glass behavior. ${\ensuremath{\chi}}_{\mathrm{ac}}$ is found to follow the critical slowing down law $(\ensuremath{\tau}∕{\ensuremath{\tau}}_{0}={\ensuremath{\epsilon}}^{\ensuremath{-}z\ensuremath{\nu}})$. The magnetic hysteresis loops indicate a field-induced irreversible ferromagnetic phase due to the presence of ``weak'' antiferromagnetic domains in the samples with $x\ensuremath{\geqslant}0.10$. This typical behavior is found to be most prominent in the sample with $x=0.15$. The observed cluster spin-glass state is explained by considering the interactions between the ferromagnetic and antiferromagnetic clusters in these doped systems having disorder and reduced geometrical tolerance factor $t$, which resulted from the random substitution of Mn with Cd ions.

The magnetic field dependence of low temperature specific heat in ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ was measured and analyzed using various pairing order parameters. At a zero magnetic field, the two-gap model, which has been successfully applied to ${\mathrm{MgB}}_{2}$ and the point-node model, appear to describe the superconducting gap function of ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ better than other models based on the isotropic $s$-wave, the $d$-wave line nodes, or the $s+g$ wave. The two energy gaps, ${\ensuremath{\Delta}}_{L}=2.67\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and ${\ensuremath{\Delta}}_{S}=1.19\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, are obtained. The observed nonlinear field dependence of the electronic specific heat coefficient, $\ensuremath{\gamma}(H)\ensuremath{\sim}{H}^{0.47}$, is quantitatively close to the $\ensuremath{\gamma}(H)\ensuremath{\sim}{H}^{0.5}$ expected for nodal superconductivity or that can be qualitatively explained using a two-gap scenario. Furthermore, the positive curvature in ${H}_{c2}(T)$ near ${T}_{c}$ is qualitatively similar to that in the other two-gap superconductor ${\mathrm{MgB}}_{2}$.

Comprehensive low-temperature specific-heat data $C(T,H)$ of ${\mathrm{Na}}_{0.35}{\mathrm{CoO}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}1.3\phantom{\rule{0.3em}{0ex}}{\mathrm{H}}_{2}\mathrm{O}$ with temperature $T$ down to $0.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and the magnetic field $H$ up to $8\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ are presented. In the normal state, the values of ${\ensuremath{\gamma}}_{\mathrm{n}}=13.94\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕\mathrm{mol}\phantom{\rule{0.3em}{0ex}}{\mathrm{K}}^{2}$ and Debye temperature ${\ensuremath{\Theta}}_{\mathrm{D}}=362\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are reported. At zero field, a very sharp superconducting anomaly was observed at ${T}_{c}=4.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with $\ensuremath{\Delta}C∕{\ensuremath{\gamma}}_{\mathrm{ns}}{T}_{c}=1.45$ if the specific-heat jump is normalized to the superconducting volume fraction, which is estimated to be 47.4% based on the consideration of entropy balance at ${T}_{c}$ for the second-order superconducting phase transition. In the superconducting state, the electronic contribution ${C}_{es}$ at $H=0$ can be well described by the model of the line nodal order parameter. In low $H$, $\ensuremath{\delta}\ensuremath{\gamma}(H)\ensuremath{\propto}{H}^{1∕2}$, which is also a manifestation of the line nodes. The behaviors of both ${T}_{c}(H)$ and $\ensuremath{\gamma}(H)$ suggest the anisotropy of ${H}_{c2}$, or possible crossovers or transitions occurring in the mixed state.