Magnetization study on the field-induced quantum critical point in YbRh_2Si_2

We study the field-induced quantum critical point (QCP) in YbRh$_2$Si$_2$ by low-temperature magnetization, $M(T)$, and magnetic Gr\"uneisen ratio, $\Gamma_{\rm mag}$, measurements and compare the results with previous thermal expansion, $\beta(T)$, and critical Gr\"uneisen ratio, $\Gamma^{cr}(T)$, data on YbRh$_2$(Si$_{0.95}$Ge$_{0.05}$)$_2$. In the latter case, a slightly negative chemical pressure has been used to tune the system towards its zero-field QCP. The magnetization derivative $-dM/dT$ is far more singular than thermal expansion, reflecting a strongly temperature dependent pressure derivative of the field at constant entropy, $(dH/dP)_S=V_m\beta/(dM/dT)$ ($V_m$: molar volume), which saturates at $(0.15\pm 0.04)$ T/GPa for $T\to 0$. The line $T^\star(H)$, previously observed in Hall- and thermodynamic measurements, separates regimes in $T$-$H$ phase space of stronger $(\epsilon>1$) and weaker $(\epsilon<1$) divergent $\Gamma_{\rm mag}(T)\propto T^{-\epsilon}$.Comment: 4 Pages, 3 Figures, submitted to Proceedings of ICM 2009 (Karlsruhe

Ferromagnetic quantum critical fluctuations in YbRh_2(Si_{0.95}Ge_{0.05})_2

The bulk magnetic susceptibility $\chi(T,B)$ of YbRh$_2$(Si$_{0.95}$Ge$_{0.05}$)$_2$ has been investigated %by ac-and dc-magnetometry at low temperatures and close to the field-induced quantum critical point at $B_c=0.027$ T. For $B\leq 0.05$ T a Curie-Weiss law with a negative Weiss temperature is observed at temperatures below 0.3 K. Outside this region, the susceptibility indicates ferromagnetic quantum critical fluctuations: $\chi(T)\propto T^{-0.6}$ above 0.3 K, while at low temperatures the Pauli susceptibility follows $\chi_0\propto (B-B_c)^{-0.6}$ and scales with the coefficient of the $T^2$ term in the electrical resistivity. The Sommerfeld-Wilson ratio is highly enhanced and increases up to 30 close to the critical field.Comment: Physical Review Letters, to be publishe

Divergence of the Magnetic Gr\"{u}neisen Ratio at the Field-Induced Quantum Critical Point in YbRh2_2Si2_2

The heavy fermion compound YbRh$_2$Si$_2$ is studied by low-temperature magnetization $M(T)$ and specific-heat $C(T)$ measurements at magnetic fields close to the quantum critical point ($H_c=0.06$ T, $H\perp c$). Upon approaching the instability, $dM/dT$ is more singular than $C(T)$, leading to a divergence of the magnetic Gr\"uneisen ratio $\Gamma_{\rm mag}=-(dM/dT)/C$. Within the Fermi liquid regime, $\Gamma_{\rm mag}=-G_r(H-H_c^{fit})$ with $G_r=-0.30\pm 0.01$ and $H_c^{fit}=(0.065\pm 0.005)$ T which is consistent with scaling behavior of the specific-heat coefficient in YbRh$_2$(Si$_{0.95}$Ge$_{0.05}$)$_2$. The field-dependence of $dM/dT$ indicates an inflection point of the entropy as a function of magnetic field upon passing the line $T^\star(H)$ previously observed in Hall- and thermodynamic measurements.Comment: 4 pages, 3 Figure

Magnetic phase transitions in the two-dimensional frustrated quantum antiferromagnet Cs2CuCl4

We report magnetization and specific heat measurements in the 2D frustrated spin-1/2 Heisenberg antiferromagnet Cs2CuCl4 at temperatures down to 0.05 K and high magnetic fields up to 11.5 T applied along a, b and c-axes. The low-field susceptibility chi (T) M/B shows a broad maximum around 2.8 K characteristic of short-range antiferromagnetic correlations and the overall temperature dependence is well described by high temperature series expansion calculations for the partially frustrated triangular lattice with J=4.46 K and J'/J=1/3. At much lower temperatures (< 0.4 K) and in in-plane field (along b and c-axes) several new intermediate-field ordered phases are observed in-between the low-field incommensurate spiral and the high-field saturated ferromagnetic state. The ground state energy extracted from the magnetization curve shows strong zero-point quantum fluctuations in the ground state at low and intermediate fields

Field-induced suppression of the heavy-fermion state in YbRh_2Si_2

We report DC magnetization measurements on YbRh_2Si_2 at temperatures down to 0.04K, magnetic fields B<11.5T and under hydrostatic pressure P<1.3GPa. At ambient pressure a kink at B*=9.9T indicates a new type of field-induced transition from an itinerant to a localized 4f-state. This transition is different from the metamagnetic transition observed in other heavy fermion compounds, as here ferromagnetic rather than antiferromagnetic correlations dominate below B*. Hydrostatic pressure experiments reveal a clear correspondence of B* to the characteristic spin fluctuation temperature determined from specific heat

Low Temperature Thermodynamic Properties of the Heavy Fermion Compound YbAgGe Close to the Field-Induced Quantum Critical Point

We present temperature and field dependent heat capacity and magnetization data taken at temperatures down to 50 mK and in an applied magnetic field up to 11.5 Tesla for YbAgGe, a heavy-fermion compound with a field induced quantum critical point. These data clearly indicate that the same electronic degrees of freedom are responsible for the features seen in both specific heat and magnetization data. In addition, they further refine the different boundaries suggested for the H - T phase diagram of YbAgGe through previous, magneto-transport measurements, and allow for further understanding of different phases on the H - T phase diagram, in particular, clearly disconnecting the field-induced quantum critical point in YbAgGe from any sort of saturation of the Yb moment in higher applied magnetic field

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

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