Field-induced quantum critical point in CeCoIn5

The resistivity of the heavy-fermion superconductor CeCoIn5 was measured as a function of temperature, down to 25 mK and in magnetic fields of up to 16 T applied perpendicular to the basal plane. With increasing field, we observe a suppression of the non-Fermi liquid behavior, rho approximately T, and the development of a Fermi liquid state, with its characteristic rho=rho(0)+AT2 dependence. The field dependence of the T2 coefficient shows critical behavior with an exponent of 1.37. This is evidence for a field-induced quantum critical point (QCP), occurring at a critical field which coincides, within experimental accuracy, with the superconducting critical field H(c2). We discuss the relation of this field-tuned QCP to a change in the magnetic state, seen as a change in magnetoresistance from positive to negative, at a crossover line that has a common border with the superconducting region below approximately 1 K.     

Possible Fulde-Ferrell-Larkin-Ovchinnikov superconducting state in CeCoIn5

We report specific heat measurements of the heavy fermion superconductor CeCoIn5 in the vicinity of the superconducting critical field H(c2), with magnetic fields in the [110], [100], and [001] directions, and at temperatures down to 50 mK. The superconducting phase transition changes from second to first order for fields above 10 T for H parallel [110] and H parallel [100]. In the same range of magnetic fields, we observe a second specific heat anomaly within the superconducting state. We interpret this anomaly as a signature of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting state. We obtain similar results for H parallel [001], with the FFLO state occupying a smaller part of the phase diagram.     

Superconductivity in CeCoIn5-xSnx: veil over an ordered state or novel quantum critical point?

Measurements of specific heat and electrical resistivity in magnetic fields up to 9 T along [001] and temperatures down to 50 mK of Sn-substituted CeCoIn5 are reported. The maximal -ln(T) divergence of the specific heat at the upper critical field Hc2 down to the lowest temperature characteristic of non-Fermi-liquid systems at the quantum critical point (QCP), the universal scaling of the Sommerfeld coefficient, and agreement of the data with spin-fluctuation theory provide strong evidence for quantum criticality at Hc2 for all x< or =0.12 in CeCoIn5-xSnx. These results indicate the "accidental" coincidence of the QCP located near Hc2 in pure CeCoIn5, in actuality, constitute a novel quantum critical point associated with unconventional superconductivity.     

Thermal conductivity in the vicinity of the quantum critical end point in Sr3Ru2O7

Thermal conductivity of Sr3Ru2O7 was measured down to 40 mK and at magnetic fields through the quantum critical end point at Hc=7.85 T. A peak in the electrical resistivity as a function of the field was mimicked by the thermal resistivity. In the limit as T-->0 K, we find that the Wiedemann-Franz law is satisfied to within 5% at all fields, implying that there is no breakdown of the electron despite the destruction of the Fermi liquid state at quantum criticality. A significant change in disorder [from rho0(H=0 T)=2.1 to 0.5 microOmega cm] does not influence our conclusions. At finite temperatures, the temperature dependence of the Lorenz number is consistent with ferromagnetic fluctuations causing the non-Fermi liquid behavior as one would expect at a metamagnetic quantum critical end point.     

Probing the quantum critical behavior of CeCoIn5 via Hall effect measurements

We present highly sensitive Hall effect measurements of the heavy fermion compound CeCoIn5 down to temperatures of 55 mK. A pronounced dip in the differential Hall coefficient | partial differential rho(xy)/ partial differential H| at low temperature and above the upper critical field of superconductivity, H(c2), is attributed to critical spin fluctuations associated with the departure from Landau Fermi liquid behavior. This identification is strongly supported by a systematic suppression of this feature at elevated pressures. The resulting crossover line in the field-temperature phase diagram favors a field induced quantum critical point at mu(0)H(qc) approximately 4.1 T below H(c2)(T=0) suggesting related, yet separate, critical fields.     

Correlated electron state in Ce(1-x)Yb(x)CoIn5 stabilized by cooperative valence fluctuations

X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements on Ce(1-x)Yb(x)CoIn5 (0≤x≤1) reveal that many of the characteristic features of the x=0 correlated electron state are stable for x≤0.775 and that phase separation occurs for x>0.775. The stability of the correlated electron state is apparently due to cooperative behavior of the Ce and Yb ions, involving their unstable valences. Low-temperature non-Fermi liquid behavior is observed and varies with x, even though there is no readily identifiable quantum critical point. The superconducting critical temperature T(c) decreases linearly with x towards 0 K as x→1, in contrast with other HF superconductors where T(c) scales with T(coh).     

Energy scales and magnetoresistance at a quantum critical point

The magnetoresistance (MR) of CeCoIn₅ is notably different from that in many conventional metals. We show that a pronounced crossover from negative to positive MR at elevated temperatures and fixed magnetic fields is determined by the scaling behavior of quasiparticle effective mass. At a quantum critical point (QCP) this dependence generates kinks (crossover points from fast to slow growth) in thermodynamic characteristics (like specific heat, magnetization, etc.) at some temperatures when a strongly correlated electron system transits from the magnetic field induced Landau-Fermi liquid (LFL) regime to the non-Fermi liquid (NFL) one taking place at rising temperatures. We show that the above kink-like peculiarity separates two distinct energy scales in QCP vicinity - low temperature LFL scale and high temperature one related to NFL regime. Our comprehensive theoretical analysis of experimental data permits to reveal for the first time new MR and kinks scaling behavior as well as to identify the physical reasons for above energy scales.     

Universal scaling and a novel quantum critical behavior of CeRhSb1-xSnx

We observe and explain a universal scaling rhochi = const for the electrical resistivity rho with the inverse magnetic susceptibility chi(-1) for the Kondo insulator CeRhSb(1-x)Snx. In the regime where the Kondo gap disappears (x > 0.12), the system forms a non-Fermi liquid (NFL), which transforms into a Fermi liquid at higher temperature. The NFL behavior is associated with the presence of a novel quantum critical point (QCP) at the Kondo insulator-correlated metal boundary. The divergent behavior of the resistivity, the susceptibility, and the specific heat has been observed when approaching the QCP from the metallic side and is interpreted as due to the competition between the Kondo and the intersite magnetic correlations.     

Possible Fulde-Ferrell-Larkin-Ovchinnikov inhomogeneous superconducting state in CeCoIn5

We present specific heat and thermal conductivity of the heavy fermion superconductor CeCoIn5 in the vicinity of the superconducting critical fieldH _c2, measured with magnetic field in the plane of this quasi-2D compound and at temperatures down to 50 mK. The superconducting phase diagram and the first order nature of the superconducting phase transition at high fields close to a critical fieldH _c2 indicate the importance of the Pauli limiting effect in CeCoIn₅. In the same range of magnetic field we observe a second specific heat anomaly within the superconducting state, and interpret it as a signature of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting state. In addition, the thermal conductivity data as a function of field display a kink at a fieldH _k below the superconducting critical field, which closely coincides with the low temperature anomaly in specific heat tentatively identified with the appearance of the FFLO superconducting state. The enhancement of thermal conductivity within the FFLO state calls for further theoretical investigations of the real space structure of the order parameter (and in particular, the structure of vortices) and of the thermal transport within the inhomogeneous FFLO state.     

Quantum critical behavior in an antiferromagnetic heavy-fermion Kondo lattice system(Ce_(1-x)La_x)_2Ir_3Ge_5

The measurements on temperature dependences of magnetic susceptibility χ(T), specific heat C(T), and electrical resistivity ρ(T) were carried out for the antiferromagnetic(AFM)(Ce_(1-x)La_x)_2Ir_3Ge_5(0 ≤ x ≤ 0.66) system. It was found that the Neel temperature TNdecreases with increasing La content x, and reaches 0 K near a critical content xcr =0.6. A new phase diagram was constructed based on these measurements. A non-Fermi liquid behavior in ρ(T) and a log T relationship in C(T) were found in the samples near xcr, indicating them to be near an AFM quantum critical point(QCP) with strong spin fluctuation. Our finding indicates that(Ce_(1-x)La_x)_2Ir_3Ge_5 may be a new platform to search for unconventional superconductivity.     

Heat Capacity Under Pressure of CeCoIn 5

Among heavy-fermion (HF) superconductors, CeCoIn 5 exhibits a record high value of T c =2.3 K at ambient pressure [1]. CeCoIn 5 belongs to a new class of HF-superconductors that crystallize in the tetragonal HoCoGa 5 -structure. This structure can be regarded as alternating layers of CeIn 3 and CoIn 2 . Bulk CeIn 3 undergoes a transition from an antiferromagnetic (AFM) state at ambient pressure ( T N =10.2 K) to a superconducting state with very low T C =0.15 K at a critical pressure p c =2.8 GPa [2] at which long range magnetic order vanishes. It is, therefore, regarded as a possible candidate for magnetically mediated superconductivity (SC). We report on measurements of the heat capacity of CeCoIn 5 at hydrostatic pressures p h 1.5 GPa. While T c increases with increasing pressure, the effective mass of the quasi-particles m eff decreases, as indicated by the ratio C / T | T c . As a working hypothesis based on theories of a nearly antiferromagnetic Fermi-liquid (NAFFL), this may be interpreted as the stabilization of the superconducting state by an increase of the characteristic spin fluctuation temperature T_{SF} (T_{SF}\propto k_F^2/m_{\rm eff}).     

Universal behavior of heavy-fermion metals near a quantum critical point

The behavior of the electronic system of heavy-fermion metals is considered. We show that there exist at least two main types of the behavior when the system is near quantum critical point, which can be identified as the fermion condensation quantum phase transition (FCQPT). We show that the first type is represented by the behavior of a highly correlated Fermi liquid, while the second type is depicted by the behavior of a strongly correlated Fermi liquid. If the system approaches FCQPT from the disordered phase, it can be viewed as a highly correlated Fermi liquid which at low temperatures exhibits the behavior of Landau Fermi liquid (LFL). At higher temperatures T, it demonstrates the non-Fermi liquid (NFL) behavior which can be converted into the LFL behavior by the application of magnetic fields B. If the system has undergone FCQPT, it can be considered as a strongly correlated Fermi liquid which demonstrates the NFL behavior even at low temperatures. It can be turned into LFL by applying magnetic fields B. We show that the effective mass M* diverges at the very point that the Neél temperature goes to zero. The B-T phase diagrams of both liquids are studied. We demonstrate that these B-T phase diagrams have a strong impact on the main properties of heavy-fermion metals, such as the magnetoresistance, resistivity, specific heat, magnetization, and volume thermal expansion.     

Towards the identification of a quantum critical line in the (p, B) phase diagram of CeCoIn5 with thermal-expansion measurements

The low-temperature thermal expansion of CeCoIn(5) single crystals measured parallel and perpendicular to magnetic fields B oriented along the c axis yields the volume thermal-expansion coefficient β. Considerable deviations of β(T) from Fermi-liquid behavior occur already within the superconducting region of the (B, T) phase diagram and become maximal at the upper critical field B(c2)(0). However, β(T) and the Grüneisen parameter Γ are incompatible with a quantum critical point at B(c2)(0), but allow for a quantum criticality shielded by superconductivity and extending to negative pressures for B相似文献   

Unconventional superconductivity in CeIrIn5 and CeCoIn5: specific heat and thermal conductivity studies

Low temperature specific heat and thermal conductivity measurements on the ambient pressure heavy fermion superconductors CeIrIn5 and CeCoIn5 reveal power law temperature dependences of these quantities below T(c). The low temperature specific heat in both CeIrIn5 and CeCoIn5 includes T2 terms, consistent with the presence of nodes in the superconducting energy gap. The thermal conductivity data present a T-linear term consistent with the universal limit (CeIrIn5), and a low temperature T3 variation in the clean limit (CeCoIn5), also in accord with prediction for an unconventional superconductor with lines of nodes.     

Superconductivity in the americium metal as a function of pressure: probing the Mott transition

High-pressure measurements of the resistivity of americium metal are reported to 27 GPa and down to temperatures of 0.4 K. The unusual dependence of the superconducting temperature (T(c)) on pressure is deduced. The critical field [H(c)(0) extrapolated to T=0] increases dramatically from 0.05 to approximately 1 T as the pressure is increased, suggesting that the type of superconductivity is changing as pressure increases. At pressures of approximately 16 GPa the 5f electrons of Am are changing from localized to itinerant, and the crystal structure also transforms to a complex one. The role of a Mott-type transition in the development of the peak in T(c) above 16 GPa is postulated.     

Anisotropic effect of Cd and Hg doping on the Pauli limited superconductor CeCoIn5

We studied the effect of impurity on the first order superconducting (SC) transition and the high field-low temperature (HFLT) SC state of CeCoIn5 by measuring the specific heat of CeCo(In1-xCdx)_{5} with x=0.0011, 0.0022, and 0.0033 and CeCo(In1-xHgx)_{5} with x=0.000 16, 0.000 32, and 0.000 48 at temperatures down to 0.1 K and fields up to 14 T. Cd substitution rapidly suppresses the crossover temperature T0, where the SC transition changes from second to first order, to T=0 K with x=0.0022 for H parallel[100], while it remains roughly constant up to x=0.0033 for H parallel[001]. The associated anomaly of the proposed FFLO state in Hg-doped samples is washed out by x=0.000 48, while remaining at the same temperature, indicating high sensitivity of that state to impurities. We interpret these results as supporting the nonmagnetic, possibly FFLO, origin of the HFLT state in CeCoIn5.     

First-order superconducting phase transition in CeCoIn5

The superconducting phase transition in heavy fermion CeCoIn5 (T(c)=2.3 K in zero field) becomes first order when the magnetic field H parallel [001] is greater than 4.7 T, and the transition temperature is below T0 approximately 0.31T(c). The change from second order at lower fields is reflected in strong sharpening of both specific heat and thermal expansion anomalies associated with the phase transition, a strong magnetocaloric effect, and a steplike change in the sample volume. This effect is due to Pauli limiting in a type-II superconductor, and was predicted theoretically in the mid-1960s.     

Strong magnetic fluctuations in a superconducting state of CeCoIn5

We show results on the vortex core dissipation through current-voltage measurements under applied pressure and magnetic field in the superconducting phase of CeCoIn{5}. We find that as soon as the system becomes superconducting, the vortex core resistivity increases sharply as the temperature and magnetic field decrease. The sharp increase in flux-flow resistivity is due to quasiparticle scattering on critical antiferromagnetic fluctuations. The strength of magnetic fluctuations below the superconducting transition suggests that magnetism is complementary to superconductivity and therefore must be considered in order to fully account for the low-temperature properties of CeCoIn{5}.     

Pressure Studies of the Unconventional Superconductors CeTIn 5 (T: Co,Ir)

In the quest for new superconductor compounds which adopt the superconducting state at increasingly higher transition temperatures T c , a non-phonon mediated coupling between the charge carriers seems to play a key role. In order to enhance our understanding of such unconventional coupling mechanisms, we studied a new family of heavy fermion (HF) superconductors CeTIn 5 (T: transition metal) whose properties point toward the realization of unconventional superconductivity (SC): the specific heat, thermal conductivity and nuclear spin-lattice relaxation rate of CeIrIn 5 and CeCoIn 5 decrease as a power law of temperature instead of exponentially for T < T c . We report on measurements of the heat capacity of CeIrIn 5 and CeCoIn 5 at hydrostatic pressures p h 1.6 GPa. In both compounds, T c increases with increasing pressure, while the mass of the quasi-particles m eff decreases, as indicated by the ratio C / T | T c . As a working hypothesis based on theories of a nearly antiferromagnetic Fermi-liquid (NAFFL), this may be interpreted as the stabilization of the superconducting state by an increase of the characteristic spin fluctuation temperature T_{\rm SF}\ (T_{\rm SF}\propto k_{\rm F}^{2}/m_{\rm eff}).     

Texture in the superconducting order parameter of CeCoIn5 revealed by nuclear magnetic resonance

We present a 115In NMR study of the quasi-two-dimensional heavy-fermion superconductor CeCoIn5 believed to host a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state. In the vicinity of the upper critical field and with a magnetic field applied parallel to the ab plane, the NMR spectrum exhibits a dramatic change below T*(H) which well coincides with the position of reported anomalies in specific heat and ultrasound velocity. We argue that our results provide the first microscopic evidence for the occurrence of a spatially modulated superconducting order parameter expected in a FFLO state. The NMR spectrum also implies an anomalous electronic structure of vortex cores.