A low-noise series-array Josephson junction parametric amplifier

We have obtained parametric gain at 19 GHz from a distributed Josephson junction parametric amplifier whose active gain medium consists of a series array of 1000 Josephson junctions embedded in a coplanar waveguide. When cooled to 1.7 K the amplifier provides 16 dB gain in a mode where the internally generated double sideband noise referred to input is 0.5 ± 0.1 K. This noise is consistent with Nyquist noise generated from the losses. An instantaneous bandwidth of 125 MHz has been observed with a peak gain of 12 dB. The 3 dB compression point with a peak gain of 14.6 dB is -90.5 dB and the dynamic range is 38 dB

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 Below $H_{c2}$ the AF phase which competes with the paramagnetic ground state is superseded by the superconducting transition.Comment: 5 pages, 3 figure

Theory of the Transition at 0.2 K in Ni-doped Bi2Sr2CaCu2O8

A theory is put forward that the electronic phase transition at 0.2 K in Ni-doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ is result of the formation of a spin density wave in the system of Ni impurities. The driving force for the transition is the exchange interaction between the impurity spins and the spins of the conduction electrons. This creates a small gap at two of the four nodes of the superconducting gap. The effect is to reduce the thermal conductivity by a factor of two, as observed.Comment: 10 pages and 1 figur

Effect of hydrostatic pressure on the ambient pressure superconductor CePt_3Si

We studied the evolution of superconductivity (sc) and antiferromagnetism (afm) in the heavy fermion compound CePt_3Si with hydrostatic pressure. We present a pressure-temperature phase diagram established by electrical transport measurements. Pressure shifts the superconducting transition temperature, T_c, to lower temperatures. Antiferromagnetism is suppressed at a critical pressure P_c=0.5 GPa.Comment: 2 pages, 2 figures, proceedings SCES'0

Superconductivity in CeCoIn5-xSnx: Veil Over an Ordered State or Novel Quantum Critical Point?

Measurements of specific heat and electrical resistivity in magnetic fields up to 9 T along [001] and temperatures down to 50 mK of Sn-substituted CeCoIn5 are reported. The maximal -ln(T) divergence of the specific heat at the upper critical field H_{c2} down to the lowest temperature characteristic of non-Fermi liquid systems at the quantum critical point (QCP), the universal scaling of the Sommerfeld coefficient, and agreement of the data with spin-fluctuation theory, provide strong evidence for quantum criticality at H_{c2} for all x < 0.12 in CeCoIn5-xSnx. These results indicate the "accidental" coincidence of the QCP located near H_{c2} in pure CeCoIn5, in actuality, constitute a novel quantum critical point associated with unconventional superconductivity.Comment: 12 pages, 4 figure

Heat Capacity Measurements in Pulsed Magnetic Fields

The new NHMFL 60T quasi-continuous magnet produces a flat-top field for a period of 100 ms at 60 Tesla, and for longer time at lower fields, e.g. 0.5 sec at 35 Tesla. We have developed for the first time the capability to measure heat capacity at very high magnetic fields in the NHMFL 60 T quasi-continuous magnet at LANL, using a probe built out of various plastic materials. The field plateau allows us to utilize a heat-pulse method to obtain heat capacity data. Proof-of-principle heat capacity experiments were performed on a variety of correlated electron systems. Both magnet performance characteristics and physical properties of various materials studied hold out a promise of wide application of this new tool.Comment: 6 pages, 3 figures, World Scientific Pub. Co., to be publishe

Localized ferromagnetic resonance force microscopy in permalloy-cobalt films

We report Ferromagnetic Resonance Force Microscopy (FMRFM) experiments on a justaposed continuous films of permalloy and cobalt. Our studies demonstrate the capability of FMRFM to perform local spectroscopy of different ferromagnetic materials. Theoretical analysis of the uniform resonance mode near the edge of the film agrees quantitatively with experimental data. Our experiments demonstrate the micron scale lateral resolution in determining local magnetic properties in continuous ferromagnetic samples.Comment: 7 pages, 3 figure