Kai Grube

The heavy-fermion compound ${\mathrm{CeCu}}_{6-x}{\mathrm{Au}}_x$ has become a model system for unconventional magnetic quantum criticality. For small Au concentrations $0\le x<0.16$, the compound undergoes a structural transition from orthorhombic to monoclinic crystal symmetry at a temperature $T_s$ with $T_s\to 0$ for $x\approx 0.15$. Antiferromagnetic order sets in close to $x\approx 0.1$. To shed light on the interplay between quantum-critical magnetic and structural fluctuations we performed neutron-scattering and thermodynamic measurements on samples with $0\le x\le 0.3$. The resulting phase diagram shows that the antiferromagnetic and monoclinic phase coexist in a tiny Au concentration range between $x\approx 0.1$ and 0.15. The application of hydrostatic and chemical pressure allows us to clearly separate the transitions from each other and to explore a possible effect of the structural transition on the magnetic quantum-critical behavior. Our measurements demonstrate that at low temperatures the unconventional quantum criticality exclusively arises from magnetic fluctuations and is not affected by the monoclinic distortion.

The heavy-fermion compound CeCu$_{6-x}$Au$_x$ has become a model system for unconventional magnetic quantum criticality. For small Au concentrations $0 \leq x < 0.16$, the compound undergoes a structural transition from orthorhombic to monoclinic crystal symmetry at a temperature $T_{s}$ with $T_{s} \rightarrow 0$ for $x \approx 0.15$. Antiferromagnetic order sets in close to $x \approx 0.1$. To shed light on the interplay between quantum critical magnetic and structural fluctuations we performed neutron-scattering and thermodynamic measurements on samples with $0 \leq x\leq 0.3$. The resulting phase diagram shows that the antiferromagnetic and monoclinic phase coexist in a tiny Au concentration range between $x\approx 0.1$ and $0.15$. The application of hydrostatic and chemical pressure allows to clearly separate the transitions from each other and to explore a possible effect of the structural transition on the magnetic quantum critical behavior. Our measurements demonstrate that at low temperatures the unconventional quantum criticality exclusively arises from magnetic fluctuations and is not affected by the monoclinic distortion.

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.

In the heavily hole-doped iron-based superconductors $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($A=\text{K}$, Rb, and Cs), the electron effective mass increases rapidly with alkali-ion radius. To study the superconducting gap structure in this series, we measure the in-plane London penetration depth ${\lambda }_{ab}\left(T\right)$ in clean crystals of $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ down to low temperature $T\sim 0.1$ K. In ${\mathrm{KFe}}_2{\mathrm{As}}_2$, the superfluid stiffness ${\rho }_s^{ab}\left(T\right)={\lambda }_{ab}^2\left(0\right)/{\lambda }_{ab}^2\left(T\right)$ at low temperatures can be accounted for by the strongly band-dependent multiple gaps reported recently. Although the ${\lambda }_{ab}\left(T\right)$ in all systems exhibits similar nonexponential temperature dependence indicating nodes or small minima in the gap, we find that the quasiparticle excitations at low temperatures show a systematic suppression with increasing alkali-ion radius. A possible origin of such evolution of low-energy excitations is discussed in terms of the momentum-dependent effect of enhanced quasiparticle mass near a quantum critical point.

The iron-based superconductors $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ with $A=\mathrm{K}$, Rb, Cs exhibit large Sommerfeld coefficients approaching those of heavy-fermion systems. We have investigated the magnetostriction and thermal expansion of this series to shed light on this unusual behavior. Quantum oscillations of the magnetostriction allow identifying the band-specific quasiparticle masses which by far exceed the band-structure derived masses. The divergence of the Grüneisen ratio derived from thermal expansion indicates that with increasing volume along the series a quantum critical point is approached. The critical fluctuations responsible for the enhancement of the quasiparticle masses appear to weaken the superconducting state.

We discuss the paramagnetic and Néel-ordered phases of the Kondo lattice Hamiltonian on the two-dimensional square lattice by means of bond fermions. In the doped case we find two antiferromagnetic solutions, the first one with small ordered moment, heavy bands, and an antiferromagnetically folded large Fermi surface—i.e., including the localized spins—the second one with large ordered moment, light bands, and an antiferromagnetically folded conduction electron-only Fermi surface. The zero temperature phase diagram as a function of Kondo coupling and conduction electron density shows first- and second-order transition lines between the three different phases and agrees qualitatively with previous numerical studies. We compare to experiments on ${\mathrm{CeRh}}_{1-x}{\mathrm{Co}}_x{\mathrm{In}}_5$ and find qualitative agreement.

We present electrical resistivity and ac-susceptibility measurements of ${\text{GdTe}}_3, {\text{TbTe}}_3$, and ${\text{DyTe}}_3$ performed under pressure. An upper charge-density-wave (CDW) is suppressed at a rate of $d{T}_{\mathrm{CDW},1}/dP\sim -85\mathrm{K}/\mathrm{GPa}$. For ${\text{TbTe}}_3$ and ${\text{DyTe}}_3$, a second CDW below $T_{\mathrm{CDW},2}$ increases with pressure until it reaches the $T_{\mathrm{CDW},1}$($P$) line. For ${\text{GdTe}}_3$, the lower CDW emerges as pressure is increased above $\sim 1\text{GPa}$. As these two CDW states are suppressed with pressure, superconductivity (SC) appears in the three compounds at lower temperatures. Ac-susceptibility experiments performed on ${\text{TbTe}}_3$ provide compelling evidence for bulk SC in the low-pressure region of the phase diagram. We provide measurements of superconducting critical fields and discuss the origin of a high-pressure superconducting phase occurring above 5 GPa.