Sr2RuO4 Single Crystals Research Articles

The search for spontaneous edge currents in Sr2RuO4 mesa structures with controlled geometrical shapes

Scanning Hall microscopy has been used to search for spontaneous edge fields in geometrically shaped mesa structures etched into the ab surface of Sr2RuO4 single crystals in order to test recent theories of the direction of edge current flow as a function of facet orientation and band filling. We find no evidence for spontaneous edge fields in any of our mesa structures above our experimental noise floor of ± 25 mG. We do, however, observe pronounced vortex clustering at low fields and temperatures, consistent with the established semi-Meissner scenario whereby a long range attractive component to the vortex-vortex interaction arises due, for example, to the multiband nature of the superconductivity. We also see clear evidence for the formation of a square vortex lattice inside square mesa structures above 1.3 K. Our results are discussed in terms of recent relevant experimental results and theoretical predictions.

Spontaneous emergence of Josephson junctions in homogeneous rings of single-crystal Sr2RuO4

The chiral p-wave order parameter in Sr2RuO4 would make it a special case amongst the unconventional superconductors. A consequence of this symmetry is the possible existence of superconducting domains of opposite chirality. At the boundary of such domains, the locally suppressed condensate can produce an intrinsic Josephson junction. Here, we provide evidence of such junctions using mesoscopic rings, structured from Sr2RuO4 single crystals. Our order parameter simulations predict such rings to host stable domain walls across their arms. This is verified with transport experiments on loops, with a sharp transition at 1.5 K, which show distinct critical current oscillations with periodicity corresponding to the flux quantum. In contrast, loops with broadened transitions at around 3 K are void of such junctions and show standard Little–Parks oscillations. Our analysis demonstrates the junctions are of intrinsic origin and makes a compelling case for the existence of superconducting domains.

Vertical Line Nodes in the Superconducting Gap Structure of Sr2RuO4

There is strong experimental evidence that the superconductor Sr2RuO4 has a chiral p-wave order parameter. This symmetry does not require that the associated gap has nodes, yet specific heat, ultrasound and thermal conductivity measurements indicate the presence of nodes in the superconducting gap structure of Sr2RuO4. Theoretical scenarios have been proposed to account for the existence of accidental nodes or deep accidental minima within a p-wave state. To elucidate the nodal structure of the gap, it is essential to know whether the lines of nodes (or minima) are vertical (parallel to the tetragonal c axis) or horizontal (perpendicular to the c axis). Here, we report thermal conductivity measurements on single crystals of Sr2RuO4 down to 50 mK for currents parallel and perpendicular to the c axis. We find that there is substantial quasiparticle transport in the T = 0 limit for both current directions. A magnetic field H immediately excites quasiparticles with velocities both in the basal plane and in the c direction. Our data down to Tc/30 and down to Hc/100 show no evidence that the nodes are in fact deep minima. Relative to the normal state, the thermal conductivity of the superconducting state is found to be very similar for the two current directions, from H = 0 to H = Hc2. These findings show that the gap structure of Sr2RuO4 consists of vertical line nodes. Given that the c-axis dispersion (warping) of the Fermi surface in Sr2RuO4 varies strongly from surface to surface, the small a-c anisotropy suggests that the line nodes are present on all three sheets of the Fermi surface. If imposed by symmetry, vertical line nodes would be inconsistent with a p-wave order parameter for Sr2RuO4. To reconcile the gap structure revealed by our data with a p-wave state, a mechanism must be found that produces accidental line nodes in Sr2RuO4.

Chern structure in the Bose-insulating phase of Sr2RuO4 nanofilms

The quantum anomaly that breaks the symmetry, for example the parity and the chirality, in the quantization leads to a physical quantity with a topological Chern invariant. We report the observation of a Chern structure in the Bose-insulating phase of Sr2RuO4 nanofilms by employing electric transport. We observed the superconductor-to-insulator transition by reducing the thickness of Sr2RuO4 single crystals. The appearance of a gap structure in the insulating phase implies local superconductivity. Fractional quantized conductance was observed without an external magnetic field. We found an anomalous induced voltage with temperature and thickness dependence, and the induced voltage exhibited switching behavior when we applied a magnetic field. We suggest that there was fractional magnetic-field-induced electric polarization in the interlayer. These anomalous results are related to topological invariance. The fractional axion angle Θ = π/6 was determined by observing the topological magneto-electric effect in the Bose-insulating phase of Sr2RuO4 nanofilms.

Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures

The physical properties of a material are defined by its electronic structure. Electrons in solids are characterized by energy (ω) and momentum (k) and the probability to find them in a particular state with given ω and k is described by the spectral function A(k, ω). This function can be directly measured in an experiment based on the well-known photoelectric effect, for the explanation of which Albert Einstein received the Nobel Prize back in 1921. In the photoelectric effect the light shone on a surface ejects electrons from the material. According to Einstein, energy conservation allows one to determine the energy of an electron inside the sample, provided the energy of the light photon and kinetic energy of the outgoing photoelectron are known. Momentum conservation makes it also possible to estimate k relating it to the momentum of the photoelectron by measuring the angle at which the photoelectron left the surface. The modern version of this technique is called Angle-Resolved Photoemission Spectroscopy (ARPES) and exploits both conservation laws in order to determine the electronic structure, i.e. energy and momentum of electrons inside the solid. In order to resolve the details crucial for understanding the topical problems of condensed matter physics, three quantities need to be minimized: uncertainty* in photon energy, uncertainty in kinetic energy of photoelectrons and temperature of the sample. In our approach we combine three recent achievements in the field of synchrotron radiation, surface science and cryogenics. We use synchrotron radiation with tunable photon energy contributing an uncertainty of the order of 1 meV, an electron energy analyzer which detects the kinetic energies with a precision of the order of 1 meV and a He(3) cryostat which allows us to keep the temperature of the sample below 1 K. We discuss the exemplary results obtained on single crystals of Sr2RuO4 and some other materials. The electronic structure of this material can be determined with an unprecedented clarity.

Change in oxygen concentration in Sr2RuO4 single crystals after laser processing

In order to check the possibility of realization of high-temperature superconductivity in strontium ruthenate, we studied experimentally the influence of laser radiation on single-crystal samples of Sr2RuO4. Results obtained by the technique of electron-probe X-ray microanalysis showed an enhancement of the oxygen concentration in the near-surface layer of samples, which may most probably be caused by the change in the ruthenium valence as a result of laser treatment.

Transport Properties in a Single Domain of Microscale Sr2RuO4 Single Crystals

We report our approach to measuring the transport properties in a single chiral order parameter domain of microscale Sr2RuO4 single crystals. Sr2RuO4 single crystals were grown by the solid-phase reaction. Microscale single crystals were deposited from a dispersed liquid, and selected from the result of the chemical composition and crystallinity of the dispersed crystals. The selected crystals attached to gold electrodes show superconducting properties. We also found the anomalous current–voltage characteristics that show that induced voltage is an even function of bias current.

Magnetic rapid quench measurement in U(1) and SU(2) superconductors

We observed the power law and the critical exponent of quantum phase transitions by measuring a transient response in an Al/AlOx/Al junction of U(1) superconductors and Sr2RuO4 single crystals of SU(2) superconductors, when magnetic field decreased rapidly from more than the critical field to zero magnetic field below T = 0.1 K. In the Al/AlOx/Al junction, the result shows that the domain length ξ is a power function of the quench time TQ. The critical exponent is 0.30 ± 0.01. Moreover, in the Sr2RuO4 crystal, the result shows ξ is a power function of Tq and the critical exponent is 0.40 ± 0.01. From the results of U(1) and SU(2) superconductors, we consider the critical exponents observed by the quenching are derived by a mean-field value in charged systems. Using typical U(1) and SU(2) superconductors, we find that the magnetic quench is efficient method to study formation processes of topological defects for a quantum phase transition near T = 0.

Observation of quantum fluctuation in nano Sr2RuO4 p-wave superconductors

We observed quantum fluctuations of the superconducting phase θ in single domain Sr2RuO4 single crystals using a transport measurement. The temperature dependence of the resistivity of submicron Sr2RuO4 shows that a finite resistivity below the superconducting transition temperature of Tc = 1.69 K. The results may suggest that the finite resistivity is occurred by the flow of vortices due to quantum fluctuations of the superconducting phase θ in the same way as one-dimensional Josephson junction array systems.

Dynamical Superconducting Order Parameter Domains in Sr 2 RuO 4

We present direct evidence for complex p-wave order parameter symmetry and the presence of dynamical chiral order parameter domains of the form px +/- ipy in the ruthenate superconductor Sr2RuO4. The domain structure creates differences in the magnetic field modulation of the critical current of Josephson junctions fabricated on orthogonal faces of Sr2RuO4 single crystals. Transitions between the chiral states of a domain or the motion of domain walls separating them generates telegraph noise in the critical current as a function of magnetic field or time and is responsible for hysteresis observed in field sweeps of the critical current. The presence of such domains confirms the p-wave triplet spin and complex (broken time-reversal symmetry) nature of the superconducting pairing state in Sr2RuO4.

Floating zone growth and carrier relaxation dynamics in single crystals of Sr2RuO4 near the clean limit

Abstract We report details of floating zone growth of Strontium ruthenate Sr 2 RuO 4 single crystals near the clean limit, with a high onset T c ∼1.5 K. We identify conditions under which pure and defective phases, such as the Ru-precipitate based 3K-phase, form. Electronic properties, X-ray diffraction, electron probe microscopy and superconducting properties help us tailor different growth parameters. We also report the first studies of carrier relaxation time in Sr 2 RuO 4 using optical pump–probe reflectivity measurements down to the femtosecond scale. We find a very long relaxation time at 8 K, indicating low disorder and weak carrier-lattice coupling at low temperatures.

Galvanomagnetic properties of atomically disordered Sr2RuO4 single crystals

The effect of neutron-bombardment-induced atomic disorder on the galvanomagnetic properties of Sr{sub 2}RuO{sub 4} single crystals has been experimentally studied in a broad range of temperatures (1.7-380 K) and magnetic fields (up to 13.6 T). The disorder leads to the appearance of negative temperature coefficients for both the in-plane electric resistivity ({rho}{sub a}) and that along the c axis ({rho}{sub c}), as well as the negative magnetoresistance {delta}{rho}, which is strongly anisotropic to the magnetic field orientation (H || a and H || c), with the easy magnetization direction along the c axis and a weak dependence on the probing current direction in the low-temperature region. The experimental {rho}{sub a}(T) and {rho}{sub c}(T) curves obtained for the initial and radiation-disordered samples can be described within the framework of a theoretical model with two conductivity channels. The first channel corresponds to the charge carriers with increased effective masses ({approx}10m{sub e} , where m{sub e} is the electron mass) and predominantly electron-electron scattering, which leads to the quadratic temperature dependences of {rho}{sub a} and {rho}{sub c} . The second channel corresponds to the charge carriers with lower effective masses exhibiting magnetic scattering at low temperatures, which leads to the temperature dependence ofmore » the {rho}{sub a,c}(T) {proportional_to} 1/T type.« less

Odd-Parity Superconductivity in Sr 2 RuO 4

Phase-sensitive measurements were made on Sr2RuO4 to establish unambiguously the odd-parity pairing in this material. The critical current of Au(0.5)In(0.5)-Sr2RuO4 superconducting quantum interference devices prepared on Sr2RuO4 single crystals was found to be a maximum for devices with junctions on the same side of the crystal and a minimum for devices with junctions on opposite sides, in the limit of zero magnetic flux; these findings indicate that the phase of the superconducting order parameter in Sr2RuO4 changes by pi under inversion. This result verifies the odd-parity pairing symmetry and the formation of spin-triplet Cooper pairs in Sr2RuO4.

Low-temperature Hall effect in substitutedSr2RuO4

We report the results of a study of the Hall effect and magnetoresistance in single crystals of Sr2RuO4 in which Sr^(2+) has been substituted by La^(3+) (Sr(2-y)La(y)RuO(4)) or Ru^(4+) by Ti^(4+) (Sr(2)Ru(1-x)Ti(x)O(4)). For undoped Sr2RuO4, the purity is so high that the strong-field Hall coefficient can be measured for fields above 4 T. The conventional weak-field Hall coefficient as a function of doping shows a sharp jump and sign change at y ~ 0.01 that is unrelated to either a sharp change in Fermi-surface topography or a magnetic instability. The implications of these results are discussed.

The Low Temperature Microwave Properties of GdBa2Cu3O7−δ and Sr2RuO4

Measurements of the microwave surface resistance Rs and reactance Xs of GdBa2Cu3O7−δ and Sr2RuO4 single crystals are reported in the temperature range 0.4 to 25 K at frequencies from 10 to 20 GHz, using a hollow dielectric resonator technique. The GdBa2Cu3O7−δ measurements are interpreted in terms of antiferromagnetic alignment of the Gd spins, resulting in a Neel transition at 2.25 K, with a strongly temperature spin relaxation time both above and below the transition. Above Tc = 1.45 K, the normal state surface impedance of the suspected p-wave superconductor Sr2RuO4 agrees rather well with that expected from dc measurements; however, below Tc, we observe an unusual increase in the surface reactance, which we attribute to a long electron relaxation time. Using a two-fluid model, we obtain a normal fluid component that decreases almost linearly with temperature and extrapolates to a finite low-temperature value.

Temperature dependence of the penetration depth in Sr2RuO4: evidence for nodes in the gap function

We report measurements of the magnetic penetration depth in single crystals of Sr2RuO4 down to 0.04 K using a tunnel-diode based, self-inductive technique. We observe a power law temperature dependence below 0.8 K, with no sign of a second phase transition nor of a crossover predicted for a multiband superconductor. A power law dependence suggests that the gap function has nodes, inconsistent with candidate p-wave states. We argue that nonlocal effects, rather than impurity scattering, can explain the observed T2 dependence instead of the T-linear behavior expected for line nodes.

Linear-field dependence of the normal-state in-plane magnetoresistance ofSr2RuO4

The transverse and longitudinal in-plane magnetoresistances in the normal state of superconducting Sr2RuO4 single crystals have been measured. At low temperatures, both of them were found to be positive with a linear magnetic-field dependence above a threshold field, a result not expected from electronic band theory. We argue that such behavior is a manifestation of a novel coherent state characterized by a spin pseudo gap in the quasi-particle excitation spectrum in Sr2RuO4.

Spectral weight redistribution in Ca1−x Sr x VO3 and Sr2RuO4

Quasiparticle spectra of the Ca1−xSrxVO3 and Sr2RuO4 single-crystals are reported. The spectra have been measured around 14K for Ca1−xSrxVO3 and 80K for Sr2RuO4 by high-resolution angle-integrated photoemission spectroscopy. Large spectral weight redistribution has been observed in both systems, suggesting an importance of the electron correlation.

Sr2RuO4: A metallic substrate for the epitaxial growth of YBa2Cu3O7−δ

Single crystals of Sr2RuO4 were grown by the floating zone melting technique. The crystals have the K2NiF4 structure and display a metallic resistivity behavior in the a-b-plane between 300 and 4.2 K (ρab≂10−4 Ω cm at 300 K). The in-plane lattice mismatch between YBa2Cu3O7−δ (001) and Sr2RuO4 (001) is smaller than 1.3%, better than that to SrTiO3 {100}. Epitaxial films of YBa2Cu3O7−δ with Tc(R=0) as high as 86 K have been grown on Sr2RuO4 crystals. The epitaxial growth of YBa2Cu3O7−δ on Sr2RuO4 was revealed by four-circle x-ray diffraction as well as by transmission electron microscopy.