Session P54: Superconductivity: Thermodynamics and Phases

Magnetic field scaling of the specific heat of La$_{1.92}$Sr$_{0.08}$CuO$_{4 }$up to 30 Teslas

The catalogue of experimental data on cuprate superconductivity has thoroughly characterized multiple different energy scales. Despite this, the complicated relationships between these energy scales remain poorly understood. New specific heat data on the underdoped cuprate La$_{1.92}$Sr$_{0.08}$CuO$_{4}$, performed in magnetic fields up to 30 Teslas demonstrates how the specific heat scaling behavior evolves as the cyclotron energy approaches the thermal energy at the superconducting transition temperature, T$_{c}$, and the mean-field calculated upper critical field, known as H$_{c2}$. This scaling behavior gives new insight into the complicated relationship of the various energy scales of the cuprate superconductor in the vortex state.   

High Magnetic Field Specific Heat in Cuprate Superconductors

We present new high magnetic field specific heat results for both underdoped YBCO 6.51 and overdoped LSCO. With these measurements we show that across different families of cuprates and across the superconducting-doping phase diagram, 45T does very little to suppress the d-wave superconducting gap. We discuss possible interpretations of the specific heat data through the use of bandstructure, previous high field magnetization measurements, and proposed Fermi surface reconstruction scenarios.   

Evolution of the phase diagrams in the pseudoternary system Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$

The pseudo ternary system Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ has been used as a model system to investigate the effect of ferromagnetism (FM) on the unconventional superconductivity (SC), the high field ordered phase (HFOP), and quantum critical behavior [1], that was observed in PrOs$_4$Sb$_{12}$. SC in this system disappears near the Nd concentration x $\sim$ 0.58. Between x $\sim$ 0.33 and $\sim$ 0.58, weak FM, confirmed by the $\mu$SR experiments [2], was found to coexist with SC. In order to further inspect the possible quantum critical behavior, a power-law analysis of the temperature dependence of the electrical resistivity data was performed. Upon suppression of SC, for samples in the range 0.33 $<$ x $<$ 0.58, the power-law exponent decreases from $\sim$ 1.8 toward 1 in the temperature region below 2.5\,K, resembling non-Fermi liquid behavior. Detailed T-x, H-x, and H-T phase diagrams for various x will be discussed. \\[4pt] [1] Ho, et. al., PRB 83, 024511 (2011).\\[0pt] [2] Ho, et. al., 2010 APS March Meeting, A38.00005 (2010).   

Resonant Ultrasound study of underdoped single crystal YBCO cuprates

We measure elastic constants and ultrasound attenuation in ultra-high quality underdoped YBCO cuprate single crystal samples. We observe a phase transition at the pseudogap temperature and reveal additional phase structure inside the pseudogap phase.   

A Pre-Formed Pair Approach to the Dynamical (THz) Complex Conductivity in the Cuprates

In this talk we present a theory for $\sigma(\omega)=\sigma_1(\omega)+i\sigma_2(\omega)$ in the underdoped cuprates. Our work presumes that the pseudogap is associated with preformed pairs. We demonstrate how the puzzling extended range of finite $\sigma_2(\omega)$ above $T_c$ (implying a ``dynamical superfluid density'') arises from a new form of pair breaking contribution to $\sigma_2(\omega)$. This is only present in these moderately clean superconductors because of stronger than BCS attraction (BCS-BEC). Sum rule compatibility and good semiquantitative agreement with experiment is found.   

Origin of multiple quantum oscillation frequencies from single carrier nodal pocket in the underdoped cuprate superconductor YBCO

Quantum oscillations are measured in the underdoped high $T_{\rm c}$ cuprate superconductor YBa$_2$Cu$_3$O$_{6+x}$, revealing multiple frequencies. An extended study over a broad angular range and magnetic field range up to 95 T accompanied by detailed harmonic analysis is presented, establishing the multiple frequencies to arise from Fermi surface reconstruction yielding a single carrier nodal pocket. Scenarios are presented for the origin of multiple frequencies from this nodal pocket.   

Synthesis, $P-T$ phase diagram, and $T_c$ of $R_2Ba_4Cu_7O_{15}$ (R= Dy,Y, DyY, GdY )

The oxygen pressure - temperature phase diagrams of $R_2Ba_4Cu_{15}$ (R=Dy,Y, Er, DyY, GdY, and EuY) superconductors have been investigated in the temperature range between $850$ and $1025^{\circ}C$ and the pressure range between $1\,$ and $50\,atm.O_2$. The relative fraction of the phases: 123, 124, and 247, was determined by studying the intensity of the xray diffraction peaks of each phase. The condition at which the phase pure 247 exists was determined to be $P=10 \;atm$ and $T=1025^{\circ} C$. Under these conditions samples with larger size R = LaY, NdY, SmY, and Eu fail to form pure 247 phase. Annealing at $P=200 \; atm. O_2$ and $400 ^{\circ} C$ was used to increase oxygen content of the as-synthesized materials and to induce superconductivity. The highest transition temperatures of $70 K$ were observed for R=Y compositions.   

Pseudogap temperature along the Widom line of a first-order transition in doped Mott insulators

A state of matter with unusual physical properties, dubbed ``the pseudogap'', appears below a characteristic temperature $T^*$ in hole-doped high-temperature superconductors. In many cases, it is not even clear whether $T^*$ is a crossover or a phase transition. We construct the normal-state phase diagram of the two dimensional Hubbard model using cellular dynamical mean-field theory. We find that $T^*$ is a crossover line above the critical endpoint of a first-order phase transition between two metallic phases, one with a pseudogap and one without. Thus $T^*$ appears in a new light: it is an unexpected example of a phenomenon observed in fluids, namely a sharp crossover between different dynamical regimes along a line of thermodynamic anomalies that appears above a first-order phase transition, the Widom line. Our findings thus suggest that the critical point of a first-order transition, and not a quantum critical point, can be the organizing principle for the rich behavior of the normal state of the cuprates. Refs: G. Sordi et al., PRL 104, 226402 (2010); G. Sordi et al., PRB 84, 075161 (2011); G. Sordi et al., arXiv:1110.1392 (2011).