In-plane and out-of-plane magnetoresistance in La2-xSrxCuO4 single crystals

The Wiedemann-Franz law in doped Mott insulators without quasiparticles

Many metallic quantum materials display anomalous transport phenomena that defy a Fermi liquid description. Here, we use numerical methods to calculate thermal and charge transport in the doped Hubbard model and observe a crossover separating high- and low-temperature behaviors. Distinct from the behavior at high temperatures, the Lorenz number [Formula: see text] becomes weakly doping dependent and less sensitive to parameters at low temperatures. At the lowest numerically accessible temperatures, [Formula: see text] roughly approaches the Wiedemann-Franz constant [Formula: see text], even in a doped Mott insulator that lacks well-defined quasiparticles. Decomposing the energy current operator indicates a compensation between kinetic and potential contributions, which may help to clarify the interpretation of transport experiments beyond Boltzmann theory in strongly correlated metals.

Pseudogap in underdoped cuprate seen in longitudinal magnetoresistance

We report the results of in-plane magnetotransport study of slightly underdoped cuprate La_1.85Sr_0.15CuO₄(LSCO15) with Ni impurity. Increasing Ni contentycauses a sharp drop in longitudinal magnetoresistance (LMR) in LSCO15 to broaden and move towards higher temperatures. TemperatureTmLMR(y)of this local maximum in LMR coincides with temperatureTdev(y), below which ideal resistivity from the parallel-resistor model deviates from itsT²-dependence. A direct comparison with the hole doping evolution of pseudogap (PG) in La2-xSr_xCuO₄(LSCO), possible through the mobile-carrier concentration extracted from the thermopower measurements, allows to equate both characteristic temperaturesTmLMR≅Tdevwith PG opening temperatureT∗. The rate of PG closing by magnetic field parallel to the CuO₂plane, in measurements up to 9 T, is consistent with spin-paramagnetic effect in this configuration and yields PG closing fieldB_pcclose to the second critical fieldBc2predicted for superconducting gap with the help of Werthamer-Helfand-Hohenberg theory. The field anisotropy ofB_pcsuggests that orbital degrees of freedom also play a role in PG formation.

Anomalies in the pseudogap phase of the cuprates: competing ground states and the role of umklapp scattering

Over the past two decades, advances in computational algorithms have revealed a curious property of the two-dimensional Hubbard model (and related theories) with hole doping: the presence of close-in-energy competing ground states that display very different physical properties. On the one hand, there is a complicated state exhibiting intertwined spin, charge, and pair density wave orders. We call this 'type A'. On the other hand, there is a uniform d-wave superconducting state that we denote as 'type B'. We advocate, with the support of both microscopic theoretical calculations and experimental data, dividing the high-temperature cuprate superconductors into two corresponding families, whose properties reflect either the type A or type B ground states at low temperatures. We review the anomalous properties of the pseudogap phase that led us to this picture, and present a modern perspective on the role that umklapp scattering plays in these phenomena in the type B materials. This reflects a consistent framework that has emerged over the last decade, in which Mott correlations at weak coupling drive the formation of the pseudogap. We discuss this development, recent theory and experiments, and open issues.

Scaling of magnetotransport in the Ba(Fe1-x Co x )2As2 series

We present magnetotransport studies of the parent, an underdoped and an optimally doped composition of the Ba(Fe_1-x Co _x )₂As₂ series. We observe that both the Kohler's and modified Kohler's scaling is typically violated in these systems. A notable exception is the magnetically ordered state of the underdoped composition where the modified Kohler's scaling is observed to be valid, indicating its relative similarity to the cuprates and some heavy fermion systems. This composition also exhibits a feature in the Hall angle, which could signify the opening of a pseudogap before the onset of long range magnetic order. The implications of our observations are discussed in the context of magnetotransport of metals with incipient magnetic fluctuations.

Magnetotransport studies of optimally doped Sr(Fe1-x Co x )2As2

We report magnetotransport measurements and their scaling analysis for the optimally electron-doped Sr(Fe_0.88Co[Formula: see text]As₂ system. We observe that both the Kohler's and modified Kohler's scalings are violated. Interestingly, the Hall angle displays a quadratic temperature dependence (cot[Formula: see text] [Formula: see text] T ²) similar to many cuprates and heavy fermion systems. The fact that this T ² dependence is seen in spite of the violation of modified Kohler's scaling suggests that the Hall angle and magnetoresistance are not governed by the same scattering mechanism. We also observe a linear magnetoresistance in this system, which does not harbor a spin density wave ground state. Implications of our observations are discussed in the context of existing models for the magnetotransport of these strongly correlated electron systems.

In-plane magnetoresistance obeys Kohler's rule in the pseudogap phase of cuprate superconductors

We report in-plane resistivity (ρ) and transverse magnetoresistance (MR) measurements for underdoped HgBa(2)CuO(4+δ) (Hg1201). Contrary to the long-standing view that Kohler's rule is strongly violated in underdoped cuprates, we find that it is in fact satisfied in the pseudogap phase of Hg1201. The transverse MR shows a quadratic field dependence, δρ/ρ(0)=aH(2), with a(T)∝T(-4). In combination with the observed ρ∝T(2) dependence, this is consistent with a single Fermi-liquid quasiparticle scattering rate. We show that this behavior is typically masked in cuprates with lower structural symmetry or strong disorder effects.

Conductance anisotropy and linear magnetoresistance in La(2 - x)Sr(x)CuO4 thin films

We have performed a detailed study of conductance anisotropy and magnetoresistance (MR) of La(2 - x)Sr(x)CuO(4) (LSCO) thin films (0.10 < x < 0.25). These two observables are promising for the detection of stripes. Subtle features of the conductance anisotropy are revealed by measuring the transverse resistance R(xy) in zero magnetic field. It is demonstrated that the sign of R(xy) depends on the orientation of the LSCO Hall bar with respect to the terrace structure of the substrate. Unit-cell-high substrate step edges must therefore be a dominant nucleation source for antiphase boundaries during film growth. We show that the measurement of R(xy) is sensitive enough to detect the cubic-tetragonal phase transition of the SrTiO(3)(100) (STO) substrate at 105 K. The MR of LSCO thin films shows for 0.10 < x < 0.25 a non-monotonic temperature dependence, resulting from the onset of a linear term in the MR above 90 K. We show that the linear MR scales with the Hall resistivity as [Formula: see text], with the constant of proportionality independent of temperature. Such scaling suggests that the linear MR originates from current distortions induced by structural or electronic inhomogeneities. The possible role of stripes for both the MR and the conductance anisotropy is discussed throughout the paper.

Nernst effect measurements of epitaxial Y0.95Ca0.05Ba2(Cu1-xZnx)3Oy and Y0.9Ca0.1Ba2Cu3Oy superconducting films

We report Nernst effect data for crystalline films of Y0.95Ca0.05Ba2(Cu1-xZnx)3Oy (with x=0, 0.02, and 0.04) and Y0.9Ca0.1Ba2Cu3Oy grown by pulsed laser deposition. We show that our own results and published data for LSCO are consistent with the theory of Gaussian superconducting fluctuations. We also show that Zn doping increases the Nernst coefficient simply because it reduces the in-plane conductivity.

An alternative theory on relaxation rates in cuprate superconductors

Transport properties of high transition temperature (high-T(c)) superconductors apparently demonstrate two distinct relaxation rates in the normal state. We propose that this superficial inconsistence can be resolved with an effective carrier (quasiparticle) density n almost linear in temperature T. Experimental evidence both for and against this explanation is analyzed and we conclude that this offers a clear yet promising scenario. Band structure calculation was utilized to determine the Fermi surface topology of the cuprate superconductor versus doping. The results demonstrate that an electron-like portion of the Fermi surface exists in a wide range of doping levels even for a p-type superconductor, exemplified by La(2-x)Sr(x)CuO(4-δ) (LSCO). Such electron-like segments have also been confirmed in recent photoemission electron spectroscopy. The Coulomb interaction between electron-like and hole-like quasiparticles then forms a bound state, similar to that of an exciton. As a result the number of charge carriers upon cooling temperature is decreased. A quantum mechanical calculation of scattering cross section demonstrates that a T(2) relaxation rate is born out of an electron-hole collision process. Above the pseudogap temperature T(*) the normal state of high-T(c) cuprates is close to a two-component Fermi liquid. It, however, assumes non-Fermi-liquid behavior below T(*).

Three-dimensionality of field-induced magnetism in a high-temperature superconductor

Many physical properties of high-temperature superconductors are two-dimensional phenomena derived from their square-planar CuO2 building blocks. This is especially true of the magnetism from the copper ions. As mobile charge carriers enter the CuO2 layers, the antiferromagnetism of the parent insulators, where each copper spin is antiparallel to its nearest neighbours, evolves into a fluctuating state where the spins show tendencies towards magnetic order of a longer periodicity. For certain charge-carrier densities, quantum fluctuations are sufficiently suppressed to yield static long-period order, and external magnetic fields also induce such order. Here we show that, in contrast to the chemically controlled order in superconducting samples, the field-induced order in these same samples is actually three-dimensional, implying significant magnetic linkage between the CuO2 planes. The results are important because they show that there are three-dimensional magnetic couplings that survive into the superconducting state, and coexist with the crucial inter-layer couplings responsible for three-dimensional superconductivity. Both types of coupling will straighten the vortex lines, implying that we have finally established a direct link between technical superconductivity, which requires zero electrical resistance in an applied magnetic field and depends on vortex dynamics, and the underlying antiferromagnetism of the cuprates.

Evidence for universal signatures of Zeeman-splitting-limited pseudogaps in superconducting electron- and hole-doped cuprates

We probe the "normal" state in electron-doped (n-type) Sm2-xCexCuO4-delta through interlayer tunneling transport in magnetic fields up to 45 T. The behavior of intrinsic high-field c-axis negative magnetoresistance (MR), which is accessed in small 30 nm-high mesa structures, is characteristic of the pseudogap state. It follows a universal correlation between the excess low-energy dissipation due to the pseudogap and its closing field Hpg and is in close correspondence with the hole-doped (p-type) Bi2Sr2CaCu2O8+y. The MR in the mesas and in the bulk crystals consistently gives a Zeeman relation between the pseudogap temperature T* and its closing field, pointing to a preeminent role of spin-singlet correlations in forming the pseudogap in cuprates, regardless of their n or p type.

Gaussian superconducting fluctuations, thermal transport, and the nernst effect

We calculate the contribution of superconducting fluctuations to thermal transport in the normal state, at low magnetic fields. We do so in the Gaussian approximation to their critical dynamics which is also the Aslamazov-Larkin approximation in the microscopics. Our results for the thermal conductivity tensor and the transverse thermoelectric response are new. The latter compare favorably with the data of Ong and collaborators on the Nernst effect in the cuprates.

Magnetoresistance of untwinned YBa(2)Cu(3)O(y) single crystals in a wide range of doping: anomalous hole-doping dependence of the coherence length

Magnetoresistance (MR) in the a-axis resistivity of untwinned YBa(2)Cu(3)O(y) single crystals is measured for a wide range of doping ( y = 6.45-7.0). The y dependence of the in-plane coherence length xi(ab) estimated from the fluctuation magnetoconductance indicates that the superconductivity is anomalously weakened in the 60-K phase; this observation, together with the Hall coefficient and the a-axis thermopower data which suggest the hole doping to be 12% for y approximately equal to 6.65, gives evidence that the origin of the 60-K plateau is the 1/8 anomaly. At high temperatures, the normal-state MR data show signatures of the Zeeman effect on the pseudogap in underdoped samples.

Three-dimensional elastic compatibility and varieties of twins in martensites

We model a cubic-to-tetragonal martensitic transition by a Ginzburg-Landau free energy in the symmetric strain tensor. We show in three dimensions (3D) that solving the St. Venant compatibility relations for strain, treated as independent field equations, generates three anisotropic long-range potentials between the two order parameter components. These potentials encode 3D discrete symmetries, express the energetics of lattice integrity, and determine 3D textures. Simulation predictions include twins with temperature-varying orientation, helical twins, competing metastable states, and compatibility-induced elastic frustration. Our approach also applies to improper ferroelastics.

Metal-insulator crossover in superconducting cuprates in strong magnetic fields

The metal-insulator crossover of the in-plane resistivity upon temperature decrease, recently observed in several classes of cuprate superconductors, when a strong magnetic field suppresses the superconductivity, is explained using the U(1)xSU(2) Chern-Simons gauge field theory. The origin of this crossover is the same as that for a similar phenomenon observed in heavily underdoped cuprates without magnetic field. It is due to the interplay between the diffusive motion of the charge carriers and the "peculiar" localization effect due to short-range antiferromagnetic order. We also calculate the in-plane transverse magnetoresistance which is in fairly good agreement with available experimental data.