Point-contact properties of non-Fermi liquid compound YbCu5-xAlx (x = 1.3 - 1.75) in high magnetic fields.

   We present the results of the first application of PCS to the compound YbCu_5-xAl_x (x = 1.3 - 1.75) showing non-Fermi liquid (NFL) behavior in the vicinity of quantum critical point (QCP). YbCu_5-xAl_x is a very interesting compound with a valency change from ~ 2.2 (x = 0) to ~ 3 (x = 2), which causes a magnetic instability near a critical concentration x_cr = 1.5 in the cross-over from the almost nonmagnetic 4f¹⁴ state in YbCu₅ to the magnetic 4f¹³ state in YbCu₃Al₂. The intermetallic system YbCu_5-xAl_x with concentration in the vicinity of xcr exhibits a typical NFL behavior, like a negative logarithmic term in the temperature-dependent specific heat or deviations from the quadratic temperature dependence of the electrical resistivity. Such a NFL behavior is frequently found in strongly correlated electron systems where the magnetic ordering temperature tends towards zero leading to a QCP. In our system the QCP is near x_cr. There exist scenarios for the occurrence of NFL behavior, but the microscopic basis of the NFL ground state is not yet completely understood.
   Our previous PC experiments on YbCu_5-xAl_x (x = 1.3 - 1.6) have been preformed at temperatures down to 1.5 K and in magnetic fields up to 6 T in the hetero-contact configuration using a Cu or Pt counter-electrode. The differential resistance dV/dI(V) as a function of the applied voltage V revealed an asymmetric behavior. In the case of x_cr = 1.5 we have observed a maximum at 1.3 mV in only one voltage polarity. This new type of asymmetry is connected with the NFL behavior at the QCP. The application of a magnetic field strongly changed the shape of the differential resistance, showing a recovering of the Fermi-liquid (FL) behavior characteristic for Kondo compounds. The behavior of PC dependencies of other concentrations differ from the one at the critical concentration.    Since previous measurements were done in the magnetic fields up to 6 T only, which is not enough to fully suppress the NFL behavior and restore the FL behavior, we applied high magnetic fields up to 22 T. In this overview we present systematic point-contact measurements in the hetero- and homo-contact arrangement at low temperatures in magnetic fields up to 22 T, studying several YbCu_5-xAl_x compounds in the vicinity of QCP (x_cr = 1.5). Measurements of hetero-contacts were done down to 100 mK. We used the same samples as characterized and studied in previous work of E. Bauer.
   The dV/dI(V) curves of a YbCu_3.5Al_1.5 - Pt hetero-contact at 1.5 K are shown in Fig. 1 in the applied magnetic fields. With increasing magnetic field the characteristic maximum at 1.3 mV (only present in one polarity of the applied voltage) shifts to higher voltages and evolves subsequently into a splitted two-peak structure. In general, the point-contact resistance is decreasing with increasing magnetic field. For high magnetic fields (above 12 T) the voltage position of both peaks is symmetric with respect to zero voltage but their intensities are different. This kind of slightly asymmetric behavior (B > 12 T) is characteristic for the point-contact dV/dI(V) dependencies of heavy-fermion compounds in a magnetic field. From measurements of the electrical resistivity, magnetic susceptibility and specific heat on bulk samples of YbCu_3.5Al_1.5 results, that approximately 12 T is enough to recover FL behavior and that compounds with concentrations x < x_cr exhibit Kondo-like behavior. Therefore, we conclude that 12 T is enough to recover the FL behavior of the Kondo type in our PC spectra. This observation gives an argument for the NFL origin of our observed asymmetry in magnetic fields below about 12 T.

Figure 1: Characteristic magnetic field behavior of dV/dI(V) for hetero-contact YbCu_3.7Al_1.3 - Cu at 1.5 K.

   In order to shed light on the origin of the observed asymmetric maximum in the hetero-contacts, we performed the PC dV/dI(V) measurements in the homo-contact arrangement. In Fig. 2 the characteristic behavior of homo-contact is presented (up to 9 T). Because of differences in the magnetic forces between the contacting parts in the case of the homo-contact arrangement, the contacts were less stable in an applied magnetic field compared to the case of hetero-contacts. The main result is that the maximum in dV/dI(V) occurs for the homo-contacts at zero applied voltage. With the applied magnetic field the splitting of the maximum (symmetrically positioned around zero voltage) occurs like in case of hetero-contacts. In the case of homo-contacts for x ą x_cr we also observed a maximum in dV/dI at zero-bias voltage splitting up in an applied magnetic field. The observed asymmetry in the current-voltage characteristic does not depend on the used needle (Cu or Pt).

Figure 2: Characteristic magnetic field behavior of dV/dI(V) for homo-contact YbCu_3.5Al_1.5 - YbCu_3.5Al_1.5 at 1.5 K.

   Fig. 3 shows the temperature evolution of symmetric (a) and asymmetric (b) part of hetero-contact YbCu_3.5Al_1.5 - Cu at zero applied magnetic field. The inset shows the bulk resistivity r(T) of this compound. Also for the case of an hetero-contact, the behavior of symmetric part dV/dI(V)s does not show the corresponding decrease of the bulk r(T) (see inset) at the lowest temperatures as would be expected for the thermal heating model at the lowest bias voltages. The comparison of dV/dI(V) at 6 K and at about 100 mK is presented in Fig. 4. At lower temperatures the thermal broadening is reduced, but position of the maximum remains the same. Once more, at 100 mK the decrease in r(T) at the lowest temperatures below 4 K (bulk sample) is never observed in dV/dI(V)s at the lowest bias.

Figure 3: Characteristic temperature behavior of symmetric and asymmetric parts of dV/dI(V) for hetero-contact YbCu_3.5Al_1.5 - Cu at B = 0 T. The inset shows the bulk resistivity r(T) of this compound.

   For the case of the thermal regime, the asymmetric parts of the dV/dI(V) (see Fig. 4}) would be connected to the thermopower of YbCu_5-xAl_x and we could estimate their temperature and magnetic field behavior. Taking into account the work of Mitsuda et al. and Zlatic et al. for the thermopower of the related YbCu_5-xAg_x system and other Yb-based systems and the small magnitude of the Cu thermopower at low temperatures, one could expect a negative bulk thermopower for YbCu_5-xAl_x with a minimum at low temperatures. The voltage position of the minimum (at about 2 mV) would correspond to a temperature of about 7 K for such a minimum in the thermopower, taking the free-electron Lorenz number.

Figure 3: Characteristic dV/dI(V) of hetero-contact YbCu_3.5Al_1.5 - Pt at 6 K (solid curve) and at 0.1 K (dashed curve).

   The observed new type of asymmetry in the dV/dI(V) dependencies of hetero-contacts is connected with the NFL ground state of YbCu_5-xAl_x system. The magnetic field influence on the asymmetry confirms the suppression of the NFL state and the recovering of the FL state at sufficiently high magnetic fields. Within the thermal model of contact heating, the asymmetry follows from a specific temperature dependence of the thermo-electric voltage in the NFL state. However, the observed maximum in the contact resistance near zero-bias voltage does not fully agree with the thermal regime, where an additional minimum would be expected at the lowest voltages because of a decreasing resistivity at the lowest temperatures. It could be that, at the lowest bias voltages, a crossover from the diffusive to the thermal regime has to be considered.

G. Pristáš, M. Reiffers
E. Bauer (TU Wien, Austria)