Intermetalické zlúčeniny, silne korelované systémy
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 YbCu5-xAlx (x = 1.3 - 1.75) showing non-Fermi liquid (NFL) behavior in the vicinity of quantum critical point (QCP). YbCu5-xAlx 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 xcr = 1.5 in the cross-over from the almost nonmagnetic 4f14 state in YbCu5 to the magnetic 4f13 state in YbCu3Al2. The intermetallic system YbCu5-xAlx 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 xcr. There exist scenarios for the occurrence of NFL behavior, but the microscopic basis of the NFL ground state is not yet completely understood.
Figure 1: Characteristic magnetic field behavior of dV/dI(V) for hetero-contact YbCu3.7Al1.3 - 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 xcr 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 YbCu3.5Al1.5 - YbCu3.5Al1.5 at 1.5 K.
Fig. 3 shows the temperature evolution of symmetric (a) and asymmetric (b) part of hetero-contact YbCu3.5Al1.5 - 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 YbCu3.5Al1.5 - 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 YbCu5-xAlx 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 YbCu5-xAgx 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 YbCu5-xAlx 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 YbCu3.5Al1.5 - 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 YbCu5-xAlx 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