Session E8: Superconductivity: Transport Properties

Charge transport in the electron-doped cuprate superconductors

Recent studies of the normal-state charge transport of the hole-doped cuprates have revealed that, in the pseudogap phase at moderate doping, the behavior of the charge carriers is that of a Fermi-liquid: The scattering rate exhibits quadratic temperature and frequency dependencies [1,2], and the magnetoresistance obeys Kohler’s rule [3]. The cotangent of the Hall angle, which in a parabolic single-band model corresponds to the scattering rate, is also quadratic in temperature, and moreover doping and compound independent, and hence universal [4]. Importantly, this observable is insensitive to the opening of the pseudogap. In light of these findings, we will revisit the charge-transport in the electron-doped cuprates [5]. [1] N. Barišic et al., Proc. Natl. Acad. Sci. USA 110, 12235 (2013); [2] S. I. Mirzaei et al., Proc. Natl. Acad. Sci. USA 110, 5774 (2013); [3] M. K. Chan et al., Phys. Rev. Lett. 113, 177005 (2014); [4] N. Barišic et al., arXiv:1507.07885 (2015); [5] N. P. Armitage et al., Rev. Mod. Phys. 82, 2421 (2010).   

Multiple sign reversal of the Hall effect in electron-doped superconductor Pr$_{0.9}$LaCe$_{0.1}$CuO$_{4 \pm \delta}$ thin films

We have investigated the temperature and field dependence of the Hall resistivity of the electron-doped Pr$_{0.9}$LaCe$_{0.1}$CuO$_{4 \pm \delta}$(PLCCO) superconducting thin films($T_{c \it 0}$ =22 K). In the low magnetic field region (0.03 $\sim$ 0.1~T), a concrete triple sign reversal of the Hall resistivity $\rho_{xy}$ has been observed in the $\rho_{xy}(T)$ curve. With the increase of the magnetic field, the Hall resistivity $\rho_{xy}(T)$ suffers triple, double, single sign reversal transitions and it will be completely disappear around 4.5~T. We contribute the triple sign reversal to the competition between the hole and the electron carriers in our electron-doped samples and a fourth sign reversal may be expected in the regime of the two-band system.   

HIGH FIELD MAGNETORESISTANCE NEAR OPTIMAL DOPING IN La$_{\mathrm{2-x}}$Sr$_{\mathrm{x}}$CuO$_{\mathrm{4}}$

An outstanding experimental feature of the metallic behavior of all high-temperature superconductors near optimal doping is the linear-in-temperature resistivity observed over a wide temperature range. Although metallic quantum criticality in these systems has been proposed to be the origin of this anomalous temperature dependence, its manifestation in the magnetotransport, yet to be determined, can become a source of important insight into the physics of these compounds. The experimental challenge is to measure normal state magnetoresistance in a broad range of magnetic fields, always limited by the H$_{\mathrm{c2}}$ on the lower side and the available magnetic fields on the higher. Here we report magnetoresistance measurements in thin films of La$_{\mathrm{2-x}}$Sr$_{\mathrm{x}}$CuO$_{\mathrm{4}}$, for magnetic fields beyond 65T - well above the highest fields used in previous studies of this system. We discuss the signature for quantum criticality based upon the high-field magnetoresistance measurements over a broad temperature range.   

Fermi surface anisotropy in the cuprates

Broken rotational (C$_4$) symmetry is a distinguishing feature for a number of experiments in the underdoped high-T$_c$ cuprates, including electrical resistivity, neutron scattering, Nernst coefficient, and scanning tunneling microscopy. This broken symmetry has not been observed on the Fermi surface, however, with or without the presence of an applied magnetic field. We measure the angle-dependent magnetoresistance-a quantity known to be extremely sensitive to the geometry and symmetry of the Fermi surface-of YBa$_2$Cu$_3$O$_{6.58}$, and find that the Fermi surface has a clear two-fold symmetry, breaking the C$_4$ symmetry of the copper-oxide plane. We discuss the implications of this finding, including how it fits with recent X-ray measurements in high magnetic fields.   

Onset field for Fermi-surface reconstruction in the cuprate superconductor YBa$_{2}$Cu$_{3}$O$_{y}$

Discovery of quantum oscillations in underdoped cuprates [1] at low temperature and high magnetic field revealed the Fermi surface to contain a small closed electron pocket. It is thought to result from a reconstruction by charge order, but whether it is the order seen by NMR [2], and ultrasound [3] above a threshold field or the short-range modulations seen by X-ray diffraction in zero field is unclear [4]. Here we report measurements of the thermal Hall conductivity in YBa$_{2}$Cu$_{3}$O$_{y}$ to show that Fermi-surface reconstruction occurs only above a sharply defined onset field, equal to the transition field seen in ultrasound. This reveals that electrons do not experience long-range broken translational symmetry in the zero-field ground state. [1] Doiron-Leyraud et al., Nature 447,565 (2007) [2] Wu et al., Nature 477,191 (2011) [3] LeBoeuf et al., Nat. Phys. 9,79 (2013) [4] Ghiringhelli et al. Science 337, 821-825 (2012)   

Vortex creep and thermal depinning within strong pinning theory

Vortex pinning in type-II superconductors can occur through the collective action of many pins (weak collective pinning scenario) or through plastic deformations induced by a low density of defects (strong pinning scenario). For the latter case, a new formalism has recently be developed [1-4] to provide a quantitative link between the microscopic pinning landscape and experimentally accessible quantities describing pinning on a macroscopic level. Examples are the critical current density $j_c$, the $I$-$V$ characteristics, or the $ac$ Campbell length $\lambda_C$. Inspired by the original work of Larkin and Brazovskii [5,6] on density wave pinning, we have extended the strong pinning formalism to account for thermal depinning of flux lines and vortex creep. [1] G. Blatter, V. B. Geshkenbein, and J. A. G. Koopmann, Phys. Rev. Lett. {\bf 92}, 067009 (2004). [2] A. U. Thomann, V. B. Geshkenbein, and G. Blatter, Phys. Rev. Lett. {\bf 108}, 217001 (2012). [3] R. Willa, V. B. Geshkenbein, and G. Blatter, Phys. Rev. B {\bf 92}, 134501 (2015). [4] R. Willa, V. B. Geshkenbein, R. Prozorov and G. Blatter, Phys. Rev. Lett. (in press). [5] A. Larkin and S. Brazovskii, Solid State Communications {\bf 93}, 275 (1995). [6] S. Brazovskii and A. Larkin, Synthetic Metals {\bf 86}, 2223 (1997).   

Anomalous high-field-induced phase in underdoped La$_{1.7}$Eu$_{0.2}$Sr$_{0.1}$CuO$_{4}$

We have investigated transport properties of the underdoped, stripe-ordered La$_{1.7}$Eu$_{0.2}$Sr$_{0.1}$CuO$_{4}$ single crystals near their magnetic-field-driven superconducting transition for $0.016 \leq T$~(K)~$\leq 40$ and magnetic fields $H$ up to 18~T. At very low $T$ ($< 0.06$~K), an anomalous high-field-induced phase (HFIP) emerges in both the in-plane and out-of-plane resistivity measurements. Two temperature-independent crossing points in the magnetoresistance are identified near the boundaries of the HFIP. In addition, the HFIP exhibits signatures of glassiness, such as hysteretic behavior and memory of magnetic history. Differential resistance studies reveal nontrivial, non-Ohmic behavior, suggesting the possible presence of a vortex glass in the HFIP. A possible $H$-$T$ phase diagram for the underdoped La$_{1.7}$Eu$_{0.2}$Sr$_{0.1}$CuO$_{4}$ and the nature of the HFIP will be discussed.   

Resistance Fluctuation Spectroscopy of Charge Stripes and Intertwined Orders in the Phase Diagram of La$_{2-x}$Ba$_x$CuO$_4$

The unusual phase diagram of La$_{2-x}$Ba$_x$CuO$_4$ (LBCO) near x=1/8 doping suggests a complex intertwined relationship between high-temperature superconductivity, charge stripes, spin order, and phase coherence. The charge stripe state's short-range conductance anisotropy may be observable as fluctuations in resistance. In thin film LBCO devices grown by pulsed laser deposition, our time-resolved resistance measurements have revealed an onset of resistance noise at dopings and critical temperatures consistent with charge stripes. The phase diagram of LBCO is explored by comparing the noise onset signature of charge order to measurements of superconductivity, the Hall effect, and other phenomena. I will briefly discuss the relevance of our results in LBCO thin films and crystals to a proposed "pair-density-wave" state near x=1/8.   

Avalanches and hysteresis at the structural transition in stripe-ordered La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_{4}$

The coupling or intertwining of lattice, spin and charge orders and their effects on superconductivity are of great current interest in the physics of cuprates. The rare-earth-doped cuprate La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_{4}$ (LNSCO), for example, exhibits a first-order structural phase transition (SPT) from the low-temperature orthorhombic (LTO) to the low-temperature tetragonal (LTT) phase, with the onset of the static charge stripe order roughly coinciding with the SPT. We present out-of-plane magnetoresistance measurements around the LTO-LTT transition in LNSCO single crystals with $H\parallel c$ up to 12 T and $H \parallel ab$ up to 9 T. Hysteresis is observed for both field orientations, but for $H \parallel c$ we also find evidence for the existence of metastable states and collective dynamics in the form of avalanches and return point memory. Such behavior indicates that, in LNSCO, the LTO-LTT structural transition can be driven with $H$. A detailed analysis of the avalanche statistics is used to determine their size and field dependence, and to extract information about the domain structure and dynamics of domain walls. Our results shed light on the interplay of lattice, spin and charge degrees of freedom in stripe-ordered La-based cuprates.   

Shot noise measurement in a strongly correlated material

In strongly correlated materials, the motion of an electron is strongly affected by interactions with other electrons, leading to many interesting phenomena including metal-insulator transitions, colossal magnetoresistance, and high temperature superconductivity. Shot noise is one experimental probe for electronic correlations beyond simple electronic transport. Shot noise, which originates from the discrete nature of the charge-carrying particles, can be strongly affected by electronic correlations. Here we report initial shot noise measurements in tunnel junctions prepared from a YBa2Cu3O7-x film sample, with nanoscale junctions written by focused helium ion beam. We will discuss a comparison of the shot noise between the YBCO film sample and standard tunnel junctions, as a function of temperature and bias, and the implications of these results.   

Power-law Optical Conductivity from Unparticles: Application to the Cuprates

We calculate the optical conductivity using several models for unparticle or scale-invariant matter. Within a Gaussian action for unparticles that is gauged with Wilson lines, we find that the conductivity computed from the Kubo formalism with vertex corrections yields no non-trivial deviation from the free-theory result. This result obtains because at the Gaussian level, unparticles are just a superposition of particle fields and hence any transport property must be consistent with free theory. Beyond the Gaussian approach, we adopt the continuous mass formulation of unparticles and calculate the Drude conductivity directly. We show that unparticles in this context can be tailored to yield an algebraic conductivity that scales as $\omega^{-2/3}$ with the associated phase angle between the imaginary and real parts of $\arctan\frac{\sigma_2}{\sigma_1}=60^\circ$ as is seen in the cuprates. Our results indicate that at each frequency in the scaling regime, excitations on all energy scales contribute. Hence, incoherence is at the heart of the power-law in the optical conductivity in strongly correlated systems such as the cuprates.   

Optical Measurements of Thermal Diffusivity in Strange Metals

Thermal transport measurements of strongly correlated electronic systems provide key insight into their emerging collective behavior. For example, high-$T_c$ superconductors exhibit different regimes of unusual transport with “bad metallicity” at high temperatures, a pseudogap-dominated transport at intermediate temperatures, and the interplay with superconductivity at low temperatures.  We present optical non-contact measurements of local thermal diffusivity in such materials. In our apparatus we focus a laser spot onto the surface of the investigated sample; the laser power is then modulated to create a periodic, point-like heat source. Another laser is focused nearby on the surface where the local reflectivity is measured. Since the reflectivity is temperature-dependent, it serves as a contactless probe of temperature oscillations due to the heat source. By measuring the temperature profile on the surface of the sample as a function of modulation frequency, we extract the thermal diffusivity of the material.  We will present measurements of the temperature dependence and anisotropy of diffusivity in various strange metals, and discuss further applications of the apparatus.