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

Measurement of calcium isotopes (δ44Ca) using a multicollector TIMS technique

We propose a new“multicollector technique” for the thermal ionization mass spectrometer (TIMS) measurement of calcium (Ca) isotope ratios improving average internal statistical uncertainty of the 44Ca/40Ca measurements by a factor of 2–4 and average sample throughput relative to the commonly used “peak jumping method” by a factor of 3. Isobaric interferences with potassium (40K+) and titanium (48Ti+) or positively charged molecules like 24Mg19F+, 25Mg19F+, 24Mg16O+ and 27Al16O+ can either be corrected or are negligible. Similar, peak shape defects introduced by the large dispersion of the whole Ca isotope mass range from 40–48 atomic mass units (amu) do not influence Ca-isotope ratios. We use a 43Ca/48Ca double spike with an iterative double spike correction algorithm for precise isotope measurement

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

The population of white dwarf binaries with hot subdwarf companions

Hot subdwarfs (sdBs) are core helium-burning stars, which lost almost their entire hydrogen envelope in the red-giant phase. Since a high fraction of those stars are in close binary systems, common envelope ejection is an important formation channel. We identified a total population of 51 close sdB+WD binaries based on time-resolved spectroscopy and multi-band photometry, derive the WD mass distribution and constrain the future evolution of these systems. Most WDs in those binaries have masses significantly below the average mass of single WDs and a high fraction of them might therefore have helium cores. We found 12 systems that will merge in less than a Hubble time and evolve to become either massive C/O WDs, AM\,CVn systems, RCrB stars or even explode as supernovae type Ia.Comment: 5 pages, 2 figures, to appear in the proceedings of the 19th European White Dwarf Workshop, ASP Conf. Se

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