M. M. Altarawneh

Metal-insulator quantum critical point beneath the high Tc superconducting dome

An enduring question in correlated systems concerns whether superconductivity is favored at a quantum critical point (QCP) characterized by a divergent quasiparticle effective mass. Despite such a scenario being widely postulated in high Tc cuprates and invoked to explain non-Fermi liquid transport signatures, experimental evidence is lacking for a critical divergence under the superconducting dome. We use ultrastrong magnetic fields to measure quantum oscillations in underdoped YBa2Cu3O6+x, revealing a dramatic doping-dependent upturn in quasiparticle effective mass at a critical metal-insulator transition beneath the superconducting dome. Given the location of this QCP under a plateau in Tc in addition to a postulated QCP at optimal doping, we discuss the intriguing possibility of two intersecting superconducting subdomes, each centered at a critical Fermi surface instability.

Normal-state nodal electronic structure in underdoped high-Tc copper oxides

An outstanding problem in the field of high-transition-temperature (high-Tc) superconductivity is the identification of the normal state out of which superconductivity emerges in the mysterious underdoped regime. The normal state uncomplicated by thermal fluctuations can be studied using applied magnetic fields that are sufficiently strong to suppress long-range superconductivity at low temperatures. Proposals in which the normal ground state is characterized by small Fermi surface pockets that exist in the absence of symmetry breaking have been superseded by models based on the existence of a superlattice that breaks the translational symmetry of the underlying lattice. Recently, a charge superlattice model that positions a small electron-like Fermi pocket in the vicinity of the nodes (where the superconducting gap is minimum) has been proposed as a replacement for the prevalent superlattice models that position the Fermi pocket in the vicinity of the pseudogap at the antinodes (where the superconducting gap is maximum). Although some ingredients of symmetry breaking have been recently revealed by crystallographic studies, their relevance to the electronic structure remains unresolved. Here we report angle-resolved quantum oscillation measurements in the underdoped copper oxide YBa2Cu3O6 + x. These measurements reveal a normal ground state comprising electron-like Fermi surface pockets located in the vicinity of the nodes, and also point to an underlying superlattice structure of low frequency and long wavelength with features in common with the charge order identified recently by complementary spectroscopic techniques.

Magnetic Quantum Oscillations in YBa2Cu3O6.61 and YBa2Cu3O6.69 in Fields of Up to 85 T: Patching the Hole in the Roof of the Superconducting Dome

We measure magnetic quantum oscillations in the underdoped cuprates YBa2Cu3O6+x with x=0.61, 0.69, using fields of up to 85 T. The quantum-oscillation frequencies and effective masses obtained suggest that the Fermi energy in the cuprates has a maximum at hole doping p approximately 0.11-0.12. On either side, the effective mass may diverge, possibly due to phase transitions associated with the T=0 limit of the metal-insulator crossover (low-p side), and the postulated topological transition from small to large Fermi surface close to optimal doping (high p side).

Fermi-liquid behavior in an underdoped high-Tcsuperconductor

We use magnetic quantum oscillations in the underdoped high Tc superconductor YBa_2Cu_3O_(6+x) (x=0.56) measured over a broad range of temperatures 100 mK<T<18 K to extract the form of the distribution function describing the low-lying quasiparticle excitations in high magnetic fields. Despite the proximity of YBa_2Cu_3O_(6+x) (x=0.56) to a Mott insulating state, various broken symmetry ground states and/or states with different quasiparticle statistics, we find that our experimental results can be understood in terms of quasiparticle excitations obeying Fermi-Dirac statistics as in the Landau Fermi liquid theory.

Determination of anisotropic H c 2 up to 60 T in Ba 0.55 K 0.45 Fe 2 As 2 single crystals

The magnetoresistance and the radio frequency penetration depth was measured in the superconductor (Ba{sub 0.55}K{sub 0.45})Fe{sub 2}As{sub 2} under pulsed arid static magnetic fields extending to 46 tesla and down to 20 K. Using these data we are able to infer a H{sub c2}(T), H - T phase diagram, for applied fields parallel and perpendicular to the crystallographic c-axis. The upper critical field anisotropy is moderate, {approx} 3.5 close to T{sub c} , and decreases with the decrease of temperature, reaching {approx} 1.5 at T {approx} 20K. These data and analysis indicate that (i) (Ba{sub 0.55}K{sub 0.45})Fe{sub 2}As{sub 2} is well described by Ginzburg-Landau theory and at 20 K is very close to a dimensional crossover and (ii) H{sub c2}(T = 0) for this compound may easily approach fields of 75 tesla.

Proximity detector circuits: An alternative to tunnel diode oscillators for contactless measurements in pulsed magnetic field environments

A radio frequency oscillator circuit based on a proximity detector integrated circuit is described as an alternative for the traditional tunnel diode oscillator used for pulsed magnetic field measurements at low temperatures. The proximity detector circuit design, although less sensitive than tunnel diode oscillator circuits, has a number of essential advantages for measurements in the extreme environments of pulsed magnetic fields. These include the insensitivity of operation to voltages induced in the inductor coil, the elimination of a diode bias circuit and tuning, and a broad dynamic range of resonant frequency variation. The circuit has been successfully applied to measure the superconducting upper critical field in Ba(0.55)K(0.45)Fe2As2 single crystals up to 60 T.

Determination of anisotropic Hc2 up to 60 T in (Ba0.55K0.45)Fe2As2 single crystals

The magnetoresistance and the radio frequency penetration depth was measured in the superconductor (Ba{sub 0.55}K{sub 0.45})Fe{sub 2}As{sub 2} under pulsed arid static magnetic fields extending to 46 tesla and down to 20 K. Using these data we are able to infer a H{sub c2}(T), H - T phase diagram, for applied fields parallel and perpendicular to the crystallographic c-axis. The upper critical field anisotropy is moderate, {approx} 3.5 close to T{sub c} , and decreases with the decrease of temperature, reaching {approx} 1.5 at T {approx} 20K. These data and analysis indicate that (i) (Ba{sub 0.55}K{sub 0.45})Fe{sub 2}As{sub 2} is well described by Ginzburg-Landau theory and at 20 K is very close to a dimensional crossover and (ii) H{sub c2}(T = 0) for this compound may easily approach fields of 75 tesla.

Evidence for Dirac Nodes from Quantum Oscillations in SrFe2As2

We present a detailed study of quantum oscillations in the antiferromagnetically ordered pnictide compound SrFe

RF skin-depth measurement of UIrGe in high magnetic fields

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Sequential Spin Polarization of the Fermi Surface Pockets in URu 2 Si 2 and Its Implications for the Hidden Order

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