Classification of materials with divergent magnetic Gr\"uneisen parameter

At any quantum critical point (QCP) with a critical magnetic field $H_c$, the magnetic Gr\"uneisen parameter $\Gamma_{\rm H}$, which equals the adiabatic magnetocaloric effect, is predicted to show characteristic signatures such as a divergence, sign change and $T/(H-H_c)^\epsilon$ scaling. We categorize thirteen materials, ranging from heavy fermion metals to frustrated magnets, where such experimental signatures have been found. Remarkably, seven stoichiometric materials at ambient pressure show $H_c=0$. However, additional thermodynamic and magnetic experiments suggest that most of them do not show a zero-field QCP. While the existence of a pressure insensitive "strange metal" state is one possibility, for some of the materials $\Gamma_{\rm H}$ seems influenced by impurities or a fraction of moments which are not participating in a frozen state. To unambiguously prove zero-field and pressure sensitive quantum criticality, a $\Gamma_{\rm H}$ divergence is insufficient and also the Gr\"uneisen ratio of thermal expansion to specific heat must diverge.Comment: 10 pages, 6 figs, manuscript for the proceedings of SCES 201

Development of the critical exponent at the antiferromagnetic phase transition of YbRh2Si2 under chemical pressure

We present specific-heat measurements in the vicinity of the antiferromagnetic phase transition on single crystals of the alloy Yb(Rh_{1-x}Co_x)2Si2 for x<= 0.38. This study was motivated by the violation of critical universality in the undoped YbRh2Si2 (Krellner et al., Phys. Rev. Lett. 102, 196402) where we have found a large critical exponent a=0.38. For Co-doped samples we observe a drastic change of the critical fluctuations resulting in a negative a, explainable within the universality classes of phase transitions. The development of a under chemical pressure gives strong indication that the violation of critical universality in YbRh2Si2 is due to the nearby quantum critical point.Comment: Accepted for the QCNP proceedings 200

High-resolution alternating-field technique to determine the magnetocaloric effect of metals down to very low temperatures

The magnetocaloric effect or "magnetic Gr\"uneisen ratio" $\Gamma_H=T^{-1}(dT/dH)_S$ quantifies the cooling or heating of a material when an applied magnetic field is changed under adiabatic conditions. Recently this property has attracted considerable interest in the field of quantum criticality. Here we report the development of a low-frequency alternating field technique which allows to perform continuous temperature scans of $\Gamma_H(T)$ on small single crystals with very high precision and down to very low temperatures. Measurements on doped YbRh$_2$Si$_2$ show that $\Gamma_H(T)$ can be determined with this technique in a faster and much more accurate way than by calculation from magnetization and specific heat measurements

Effect of chemical substitution and pressure on YbRh2Si2

We carried out electrical resistivity experiments on (Yb,La)Rh2Si2 and on Yb(Rh,Ir)2Si2 under pressure and in magnetic fields. YbRh2Si2 exhibits a weak antiferromagnetic transition at atmospheric pressure with a N\'eel temperature of only T_N = 70 mK. By applying a small magnetic field T_N can be continuously suppressed to T=0 at B_c = 60 mT (B_|_c) driving the system to a quantum critical point (QCP). On applying external pressure the magnetic phase is stabilized and T_N(p) is increasing as usually observed in Yb-based heavy-fermion metals. Substituting Yb by La or Rh by Ir allows to create a negative chemical pressure, La (Ir) being smaller than Yb (Rh), and eventually to drive YbRh2Si2 to a pressure controlled QCP. In this paper we compare the effect of external hydrostatic pressure and chemical substitution on the ground-state properties of YbRh2Si2.Comment: 4 pages, 5 figures, proceedings paper of the QCNP0

Effect of nonmagnetic dilution in honeycomb lattice iridates Na2_2IrO3_3 and Li2_2IrO3_3

We have synthesized single crystals of Na$_2$(Ir$_{1-x}$Ti$_x$)O$_3$ and polycrystals of Li$_2$(Ir$_{1-x}$Ti$_x$)O$_3$ and studied the effect of magnetic depletion on the magnetic properties by measurements of the magnetic susceptibility, specific heat and magnetocaloric effect at temperatures down to 0.1~K. In both systems, the non-magnetic substitution rapidly changes the magnetically ordered ground state into a spin glass, indicating strong frustration. While for the Li system the Weiss temperature $\Theta_{\rm W}$ remains unchanged up to $x=0.55$, a strong decrease $|\Theta_{\rm W}|$ is found for the Na system. This suggests that only for the former system magnetic exchange beyond nearest neighbors is dominating. This is also corroborated by the observation of a smeared quantum phase transition in Li$_2$(Ir$_{1-x}$Ti$_x$)O$_3$ near $x=0.5$, i.e. much beyond the site percolation threshold of the honeycomb lattice.Comment: 8 pages including supplemental, 12 figure

Magnetic Single-Electron Transistor as a Tunable Model System for Kondo-Destroying Quantum Criticality

Single-electron transistors attached to ferromagnetic leads can undergo a continuous quantum phase transition as their gate voltage is tuned. The corresponding quantum critical point separates a Fermi liquid phase from a non-Fermi liquid one. Here, we expound on the physical idea proposed earlier. The key physics is the critical destruction of the Kondo effect, which underlies a new class of quantum criticality that has been argued to apply to heavy fermion metals. Its manifestation in the transport properties is studied through an effective Bose-Fermi Kondo model; the bosonic bath, corresponding to the spin waves of the ferromagnetic leads, describes a particular type of sub-Ohmic dissipation. We also present results for general forms of sub-Ohmic dissipative bath, and consider in some detail the case with critical paramagons replacing spin waves. Finally, we discuss some delicate aspects in the theoretical treatment of the effect of a local magnetic field, particularly in connection with the frequently employed Non-Crossing Approximation.Comment: 4 pages, 3 figures, to appear in the proceedings of SCES 07 (the international conference on strongly correlated electron systems 2007

Heavy Fermions and Quantum Phase Transitions

Quantum phase transitions arise in many-body systems due to competing interactions that promote rivaling ground states. Recent years have seen the identification of continuous quantum phase transitions, or quantum critical points, in a host of antiferromagnetic heavy-fermion compounds. Studies of the interplay between the various effects have revealed new classes of quantum critical points, and are uncovering a plethora of new quantum phases. At the same time, quantum criticality has provided fresh insights into the electronic, magnetic, and superconducting properties of the heavy-fermion metals. We review these developments, discuss the open issues, and outline some directions for future research.Comment: review article, 26 pages, 4 figure

Scaling of the magnetic entropy and magnetization in YbRh_2(Si_{0.95}Ge_{0.05})_2

The magnetic entropy of YbRh_2(Si_{0.95}Ge_{0.05})_2 is derived from low-temperature ($T\geq 18$ mK) specific heat measurements. Upon field-tuning the system to its antiferromagnetic quantum critical point unique temperature over magnetic field scaling is observed indicating the disintegration of heavy quasiparticles. The field dependence of the entropy equals the temperature dependence of the dc-magnetization as expected from the Maxwell relation. This proves that the quantum-critical fluctuations affect the thermal and magnetic properties in a consistent way.Comment: 6 pages, 2 figures, manuscript submitted to SCES2004 conferenc

Nonlinear conductivity in CaRuO3 thin films measured by short current pulses

Metals near quantum critical points have been predicted to display universal out-of equilibrium behavior in the steady current-carrying state. We have studied the non-linear conductivity of high-quality CaRuO$_3$ thin films with residual resistivity ratio up to 57 using micro-second short, high-field current pulses at low temperatures. Even for the shortest pulses of $5\mu$s, Joule heating persists, making it impossible to observe a possible universal non-linearity. Much shorter pulses are needed for the investigation of universal non-linear conductivity.Comment: 9 pages, 7 figure