Field-tuned quantum critical point of antiferromagnetic metals

A magnetic field applied to a three-dimensional antiferromagnetic metal can destroy the long-range order and thereby induce a quantum critical point. Such field-induced quantum critical behavior is the focus of many recent experiments. We investigate theoretically the quantum critical behavior of clean antiferromagnetic metals subject to a static, spatially uniform external magnetic field. The external field does not only suppress (or induce in some systems) antiferromagnetism but also influences the dynamics of the order parameter by inducing spin precession. This leads to an exactly marginal correction to spin-fluctuation theory. We investigate how the interplay of precession and damping determines the specific heat, magnetization, magnetocaloric effect, susceptibility and scattering rates. We point out that the precession can change the sign of the leading \sqrt{T} correction to the specific heat coefficient c(T)/T and can induce a characteristic maximum in c(T)/T for certain parameters. We argue that the susceptibility \chi =\partial M/\partial B is the thermodynamic quantity which shows the most significant change upon approaching the quantum critical point and which gives experimental access to the (dangerously irrelevant) spin-spin interactions.Comment: 12 pages, 8 figure

Magnetic-Field Induced Quantum Critical Point in YbRh2_2Si2_2

We report low-temperature calorimetric, magnetic and resistivity measurements on the antiferromagnetic (AF) heavy-fermion metal YbRh$_2$Si$_2$ (${T_N =}$ 70 mK) as a function of magnetic field $B$. While for fields exceeding the critical value ${B_{c0}}$ at which ${T_N\to0}$ the low temperature resistivity shows an ${AT^2}$ dependence, a ${1/(B-B_{c0})}$ divergence of ${A(B)}$ upon reducing $B$ to ${B_{c0}}$ suggests singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass. The observations are interpreted in terms of a new type of quantum critical point separating a weakly AF ordered from a weakly polarized heavy Landau-Fermi liquid state.Comment: accepted for publication in Phys. Rev. Let

Specific heat of heavy fermion CePd2Si2 in high magnetic fields

We report specific heat measurements on the heavy fermion compound CePd2Si2 in magnetic fields up to 16 T and in the temperature range 1.4-16 K. A sharp peak in the specific heat signals the antiferromagnetic transition at T_N ~ 9.3 K in zero field. The transition is found to shift to lower temperatures when a magnetic field is applied along the crystallographic a-axis, while a field applied parallel to the tetragonal c-axis does not affect the transition. The magnetic contribution to the specific heat below T_N is well described by a sum of a linear electronic term and an antiferromagnetic spin wave contribution. Just below T_N, an additional positive curvature, especially at high fields, arises most probably due to thermal fluctuations. The field dependence of the coefficient of the low temperature linear term, gamma_0, extracted from the fits shows a maximum at about 6 T, at the point where an anomaly was detected in susceptibility measurements. The relative field dependence of both T_N and the magnetic entropy at T_N scales as [1-(B/B_0)^2] for B // a, suggesting the disappearance of antiferromagnetism at B_0 ~ 42 T. The expected suppression of the antiferromagnetic transition temperature to zero makes the existence of a magnetic quantum critical point possible.Comment: to be published in Journal of Physics: Condensed Matte

Green's function for a Schroedinger operator and some related summation formulas

Summation formulas are obtained for products of associated Lagurre polynomials by means of the Green's function K for the Hamiltonian H = -{d^2\over dx^2} + x^2 + Ax^{-2}, A > 0. K is constructed by an application of a Mercer type theorem that arises in connection with integral equations. The new approach introduced in this paper may be useful for the construction of wider classes of generating function.Comment: 14 page

Future Experiments in Relativistic Heavy Ion Collisions

The measurements at RHIC have revealed a new state of matter, which needs to be further characterized in order to better understand its implications for the early evolution of the universe and QCD. I will show that, in the near future, complementary key measurements can be performed at RHIC, LHC, and FAIR. I will focus on results than can be obtained using identified particles, a probe which has been the basis for this conference over the past three decades. The sophisticated detectors, built and planned, for all three accelerator facilities enable us to measure leptons, photons, muons as well as hadrons and resonances of all flavors almost equally well, which makes these experiments unprecedented precision tools for the comprehensive understanding of the physics of the early universe.Comment: 10 pages, 4 figures, Proceedings for Summary Talk at SQM 2007, Levoca, Slovakia, June 24-29, 200

Avoided Antiferromagnetic Order and Quantum Critical Point in CeCoIn5_5

We measured specific heat and resistivity of heavy fermion CeCoIn5 between the superconducting critical field $H_{c2} = 5 T$ and 9 T, with field in the [001] direction, and at temperatures down to 50mK. At 5T the data show Non Fermi Liquid behavior down to the lowest temperatures. At field above 8T the data exhibit crossover from the Fermi liquid to a Non Fermi Liquid behavior. We analyzed the scaling properties of the specific heat, and compared both resistivity and the specific heat with the predictions of a spin-fluctuation theory. Our analysis leads us to suggest that the NFL behavior is due to incipient antiferromagnetism (AF) in CeCoIn5, with the quantum critical point in the vicinity of the $H_{c2}$. Below $H_{c2}$ the AF phase which competes with the paramagnetic ground state is superseded by the superconducting transition.Comment: 5 pages, 3 figure