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I. Magnetic, thermal and transport properties of YbCu$_{5-x}$Zn$_x$ alloys

I. Čurlik, F. Akbar, S. Gabani, M. Giovannini, J. G. Sereni

TL;DR

The study investigates $YbCu_{5-x}Zn_x$ alloys with $0.7 \le x \le 1$ to explore valence fluctuations and quantum criticality in a cubic AuBe$_5$-type lattice. Structural data reveal a lattice expansion exceeding a simple Vegard reference, pointing to a shift of Yb toward the $Yb^{2+}$ state, while magnetic measurements show a diminishing moment and a ground-state doublet with low CEF/Kondo scales. Specific heat exposes non-Fermi-liquid behavior at low $T$ and small $4f$ contributions, with well-resolved CEF splittings and Kondo-related broadening, yielding a low characteristic energy $T_q \approx 7.5$ K. Transport data exhibit single-impurity-like resistivity and a pronounced negative magnetoresistivity that scales with $\zeta = B/(T+T^*)$ and $T^* \approx 7$ K, reinforcing the presence of a small-energy-scale quantum fluctuation regime. Collectively, the results position $YbCu_{5-x}Zn_x$ as a tunable platform near a quantum critical point, with Zn content steering valence, CEF, and Kondo interactions toward or away from criticality.

Abstract

Within the family of cubic YbCu$_4$X compounds ($X$ = Ni, Au and Zn), we have investigated the YbCu$_{5-x}$Zn$_x$ ($1\geq x \geq 0.7$) alloys by means of structural, magnetic, thermal and transport measurements. In the $τ1-$ YbCu$_{5-x}$Zn$_x$ (cubic AuBe$_5$ type, $0.7 \leq x \leq 1.5$) structural phase, Yb ion is in its Yb$^{3+}$ magnetic configuration. However, by increasing Zn content the unit cell grows faster than a reference computed as a Cu by Zn atoms substitution, which indicates a shift of Yb ions towards the larger Yb$^{2+}$ configuration. The magnetic behavior confirms such tendency with a clear decrease of the saturation magnetization and effective moment between $x=0.7$ and $x=1$. The specific heat at low temperature shows a logarithmic dependence characteristic for a non-fermi-liquid behavior. The characteristic energies of all studied parameters, including magneto resistivity, show notably low values as an indication that these alloys are close to a quantum critical point, which is approached from the non-magnetic side as the Zn content decreases.

I. Magnetic, thermal and transport properties of YbCu$_{5-x}$Zn$_x$ alloys

TL;DR

The study investigates alloys with to explore valence fluctuations and quantum criticality in a cubic AuBe-type lattice. Structural data reveal a lattice expansion exceeding a simple Vegard reference, pointing to a shift of Yb toward the state, while magnetic measurements show a diminishing moment and a ground-state doublet with low CEF/Kondo scales. Specific heat exposes non-Fermi-liquid behavior at low and small contributions, with well-resolved CEF splittings and Kondo-related broadening, yielding a low characteristic energy K. Transport data exhibit single-impurity-like resistivity and a pronounced negative magnetoresistivity that scales with and K, reinforcing the presence of a small-energy-scale quantum fluctuation regime. Collectively, the results position as a tunable platform near a quantum critical point, with Zn content steering valence, CEF, and Kondo interactions toward or away from criticality.

Abstract

Within the family of cubic YbCuX compounds ( = Ni, Au and Zn), we have investigated the YbCuZn () alloys by means of structural, magnetic, thermal and transport measurements. In the YbCuZn (cubic AuBe type, ) structural phase, Yb ion is in its Yb magnetic configuration. However, by increasing Zn content the unit cell grows faster than a reference computed as a Cu by Zn atoms substitution, which indicates a shift of Yb ions towards the larger Yb configuration. The magnetic behavior confirms such tendency with a clear decrease of the saturation magnetization and effective moment between and . The specific heat at low temperature shows a logarithmic dependence characteristic for a non-fermi-liquid behavior. The characteristic energies of all studied parameters, including magneto resistivity, show notably low values as an indication that these alloys are close to a quantum critical point, which is approached from the non-magnetic side as the Zn content decreases.

Paper Structure

This paper contains 11 sections, 1 equation, 6 figures.

Table of Contents

  1. Introduction
  2. Experimental
  3. Structural properties
  4. Magnetic properties
  5. Magnetization
  6. Magnetic susceptibility
  7. Specific heat
  8. High temperature specific heat
  9. Intermediate and low temperature specific heat
  10. Electrical resistivity and Magnetoresistivity
  11. Conclusions

Figures (6)

  • Figure 1: (Color online) Comparison of the formula unit (f.u.) volume dependence with an hypotetical Vegard's law reference, where one (of five) Cu atoms is progressively substituted by one Zn. The Zn(x=0)origin is taken from YbCu5. Notice that there are four f.u. in each cell volume Akbar.
  • Figure 2: (Color online) field dependent magnetization $M(B)$ for (a) YbCu$_4$Zn and (b) YbCu$_{4.3}$Zn$_{0.7}$, up to 9T in the range of 2 K to 50 K. Continuous curves in the 2 K isotherms are the fits to extract the saturation magnetization.
  • Figure 3: (Color online) a) High temperature inverse susceptibility of YbCu$_{5-x}$Zn$_x$ (x=1 and x=0.7) alloys, up to room temperature, measured at $B=1$ T (after Akbar) where different extrapolations to $\theta_P^{ht}$ are discussed. Inset: detail of the inverse susceptibility at low temperature for all studied concentrations. b) Low temperature susceptibility in a semi-logarithmic representation, showing respective fittings. Inset: main extracted parameters: $\mu_{eff}$ (left blue axis) and $\theta_P^{lt}$ (right red axis). The left red arrow indicates the theoretic $\mu_{eff}(x)=4.54\,\mu_B$ value.
  • Figure 4: (Color online) a) High temperature specific heat of YbCu$_{5-x}Zn_x$ alloys up to room temperature in a $C_P/T$ representation, compared with a Debye function of $T_D=265$ K (dashed curve). Inset: Entropy of the $4f$ electrons after phonon subtraction. Continuous curves indicate the increase to the entropy of respective CEF levels: excited doublet (magnet) and $\Gamma_8$ quartet (dark yelow) . b) Specific heat contribution divided temperature of the $4f$ electrons in a semi-logarithmic representation indicating the NFL behavior of the GS doublet for the YbCu$_4$Zn$_1$ compound from $T\geq 0.4$ K up to 10 K. Continuous curve: fit between 0.4 and 70 K (see the text) including the CEF contribution. Inset: Comparison of the $C_{NFL}/T$ dependencies of the YbCu$_{5-x}$Zn$_x$ alloys.
  • Figure 5: (Color online) Temperature dependence of the electrical resistivity of YbCu$_4$Zn, normalized to room temperature in a semi-logarithmic representation. a) YbCu$_4$Zn compound at different applied fields. Continuous curves are fits to describe the curvature evolution at low temperature, see the text. b) Substitution of Cu by Zn in YbCu$_{5-x}$Zn$_x$
  • ...and 1 more figures