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arXiv2013

MicroSQUIDs based on V/Cu/V Josephson nanojunctions

We report on the fabrication and characterization of microSQUID devices based on nanoscale vanadium/copper/vanadium Josephson weak links. Magnetically driven quantum interference patterns have been measured for temperatures in the 0.24 - 2 K range. As magnetometers, these devices obtain flux-to-voltage transfer function values as high as 450 microV/Phi_0 leading to promising magnetic flux resolution Phi_N < 3 microPhi_0 / Hz^(1/2), being here limited by the room temperature preamplification stage. Significant improvements in the flux noise performance figures, which are already competitive with existing state-of-the-art commercial SQUIDs systems, are expected either with cryogenic preamplification or with the adoption of optimized electronic readout stages based on active feedback schemes.

cond-mat.mes-hall↗

arXiv2013

An analysis method for transmission measurements of superconducting resonators with applications to quantum-regime dielectric-loss measurements

Superconducting resonators provide a convenient way to measure loss tangents of various dielectrics at low temperature. For the purpose of examining the microscopic loss mechanisms in dielectrics, precise measurements of the internal quality factor at different values of energy stored in the resonators are required. Here, we present a consistent method to analyze a LC superconducting resonator coupled to a transmission line. We first derive an approximate expression for the transmission S-parameter $S_{21}$ based on a complete circuit model. In the weak coupling limit, we show that the internal quality factor is reliably determined by fitting the approximate form of $S_{21}$. Since the voltage $V$ of the capacitor of the LC circuit is required to determine the energy stored in the resonator, we next calculate the relation between $V$ and the forward propagating wave voltage $V_{in}^+$. Due to the dependence of the quality factor on voltage, $V$ is not simply proportional to $V_{in}^+$. We find a self-consistent way to determine the relation between $V$ and $V_{in}^+$, which employs only the fitting parameters for $S_{21}$ and a linear scaling factor. We then examine the resonator transmission in the cases of port reflection and impedance mismatch. We find that resonator transmission asymmetry is primarily due to the reflection from discontinuity in transmission lines. We show that our analysis method to extract the internal quality factor is robust in the non-ideal cases above. Finally, we show that the analysis method on LC resonator can be generalize to arbitrary weakly coupled lumped and distributed resonators. The generalization uses a systematic approximation on the response function based on the pole and zero which are the closest to the resonance frequency. This Closest Pole and Zero Method (CPZM) is a valuable tool for analyzing physical measurements of high-Q resonators.

cond-mat.mes-hall↗

arXiv2013

Jitter analysis of a superconducting nanowire single photon detector

Jitter is one of the key parameters for a superconducting nanowire single photon detector (SNSPD). Using an optimized time-correlated single photon counting system for jitter measurement, we extensively studied the dependence of system jitter on the bias current and working temperature. The signal-to-noise ratio of the single-photon-response pulse was proven to be an important factor in system jitter. The final system jitter was reduced to 18 ps by using a high-critical-current SNSPD, which showed an intrinsic SNSPD jitter of 15 ps. A laser ranging experiment using a 15-ps SNSPD achieved a record depth resolution of 3 mm at a wavelength of 1550 nm.

cond-mat.supr-con↗

arXiv2013

Spins, Stripes, and Superconductivity in Hole-Doped Cuprates

One of the major themes in correlated electron physics over the last quarter century has been the problem of high-temperature superconductivity in hole-doped copper-oxide compounds. Fundamental to this problem is the competition between antiferromagnetic spin correlations, a symptom of strong Coulomb interactions, and the kinetic energy of the doped carriers, which favors delocalization. After discussing some of the early challenges in the field, I describe the experimental picture provided by a variety of spectroscopic and transport techniques. Then I turn to the technique of neutron scattering, and discuss how it is used to determine spin correlations, especially in model systems of quantum magnetism. Neutron scattering and complementary techniques have determined the extent to which antiferromagnetic spin correlations survive in the cuprate superconductors. One experimental case involves the ordering of spin and charge stripes. I first consider related measurements on model compounds, such as La(2-x)Sr(x)NiO(4+d), and then discuss the case of La(2-x)Ba(x)CuO(4). In the latter system, recent transport studies have demonstrated that quasi-two-dimensional superconductivity coexists with the stripe order, but with frustrated phase order between the layers. This has led to new concepts for the coexistence of spin order and superconductivity. While the relevance of stripe correlations to high-temperature superconductivity remains a subject of controversy, there is no question that stripes are an intriguing example of electron matter that results from strong correlations.

cond-mat.supr-con↗

arXiv2013

Derivation of the superconducting gap equation for the noncentrosymmetric superconductor Li2Pt3B

We present here the mathematical background of our approach, presented in Phys. Rev. B 86, 134526 (2012) regarding the gap function and symmetry for the noncentrosymmetric (NCS) superconductor $Li_2Pt_3B$. As revealed by the experiment, this NCS superconductor gives rise to line nodes in the superconducting order parameter, which is responsible for many of its experimental behaviors. Owing to the enhanced d-character of the relevant bands that cross the Fermi level,the system gets weakly correlated. The nature and symmetry of this nodal behavior is explained from a microscopic viewpoint. In this article starting with an Hubbard model relevant for this NCS system by considering the effect of the onsite Coulomb repulsion on the pairing potential perturbatively, we extract the superconducting gap equation. Further analysis of this equation predicts a $s_\pm$ wave gap function with line nodes as the most promising candidate in the superconducting state.

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arXiv2013

Tunnel conductance spectroscopy via harmonic generation in a hybrid capacitor device

We address the measurement of density of states within and beyond the superconducting gap in tunnel-coupled finite-size nanostructures using a capacitive method. Third-harmonic generation is used to yield the full differential conductance spectrum without destruction of the low dimensionality otherwise induced by intimate ohmic coupling to an electrode. The method is particularly relevant to attempts to discern the presence of the fragile Majorana quasiparticle at the end of spin-orbit-coupled nanowires in appropriate magnetic field conditions by their signature mid-gap density of states.

cond-mat.mes-hall↗

arXiv2013

Anisotropic electronic properties of a-axis-oriented Sr2IrO4 epitaxial thin-films

We have investigated the transport and optical properties along the c-axis of a-axis-oriented Sr2IrO4 epitaxial thin-films grown on LaSrGaO4 (100) substrates. The c-axis resistivity is approximately one order of magnitude larger than that of the ab-plane. Optical absorption spectra with E//b polarization show both Ir 5d intersite transitions and charge-transfer transitions (O 2p to Ir 5d), while E//c spectra show only the latter. The structural anisotropy created by biaxial strain in a-axis-oriented thin-films also changes the electronic structure and gap energy. These a-axis-oriented, epitaxial thin-films provide a powerful tool to investigate the highly anisotropic electronic properties of Sr2IrO4.

cond-mat.str-el↗

arXiv2013

Planar Superconducting Whispering Gallery Mode Resonators

We introduce a microwave circuit architecture for quantum signal processing combining design principles borrowed from high-Q 3D resonators in the quantum regime and from planar structures fabricated with standard lithography. The resulting '2.5D' whispering-gallery mode resonators store 98% of their energy in vacuum. We have measured internal quality factors above 3 million at the single photon level and have used the device as a materials characterization platform to place an upper bound on the surface resistance of thin film aluminum of less than 250nOhms.

cond-mat.supr-con↗

arXiv2013

Coexistence of two superconducting phases in Ca1-xLaxFe2As2

Single crystals of Ca1-xLaxFe2As2 for x ranging from 0 to 0.25 have been grown and characterized by structural, transport and magnetic measurements. Coexistence of two superconducting phases is observed, in which the low superconducting transition temperature (Tc) phase has Tc ~ 20 K, and the high Tc phase has Tc higher than 40 K. These data also delineate an x - T phase diagram in which the single magnetic/structural phase transition in undoped CaFe2As2 appears to split into two distinct phase transitions, both of which are suppressed with increasing La substitution. Superconductivity emerges when x is about 0.06 and coexists with the structural/magnetic transition until x is ~ 0.13. With increasing concentration of La, the structural/magnetic transition is totally suppressed, and Tc reaches its maximum value of about 45 K for 0.15 < x < 0.19. A domelike superconducting region is not observed in the phase diagram, however, because no obvious over-doping region can be found. Two superconducting phases coexist in the x - T phase diagram of Ca1-xLaxFe2As2. The formation of the two separate phases, as well as the origin of the high Tc in Ca1-xLaxFe2As2 is studied and discussed in detail.

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arXiv2013

Enhancement of the Superconducting Transition Temperature in FeSe Epitaxial Thin Films by Anisotropic Compression

In order to investigate the effects of in-plane strain on the superconductivity of FeSe, epitaxial thin films of FeSe were fabricated on CaF$_2$ substrates. The films are compressed along the a-axis and their superconducting transition temperatures $T_{\mathrm c}^{\mathrm {zero}}$ reach 11.4 K, which is approximately 1.5 times higher than that of bulk crystals. The $T_{\mathrm c}$ values are weakly dependent on the ratio of the lattice constants, $c$ / $a$, compared to that of Fe(Se,Te). Our results indicate that even a binary system FeSe has room for improvement, and will open a new route for the application of Fe-based superconductors.

cond-mat.supr-con↗

arXiv2013

Modulation of conductance and superconductivity by top-gating in LaAlO3/SrTiO3 2-dimensional electron systems

We report the electrical top-gating of a 2-dimensional electron gas (2DEG) formed at the LaAlO3/SrTiO3 interface, using electron-beam evaporated Au gate electrodes. In these structures, epitaxial LaAlO3 films grown by pulsed laser deposition induce the 2DEGs at the interface to the SrTiO3 substrate and simultaneously act as the gate dielectric. The structured top-gates enable a local tuning and complete on/off switching of the interface (super)-conductivity, while maintaining the usual, intrinsic characteristics for these LaAlO3/SrTiO3 interfaces when no gate voltage is applied.

cond-mat.mtrl-sci↗

arXiv2013

Van der Waals density functional study of the structural and electronic properties of La-doped phenanthrene

By the first principles calculations based on the van der Waals density functional theory, we study the crystal structures and electronic properties of La-doped phenanthrene. Two stable atomic geometries of La$_1$phenanthrene are obtained by relaxation of atomic positions from various initial structures. The structure-\uppercase\expandafter{\romannumeral1} is a metal with two energy bands crossing the Fermi level, while the structure-\uppercase\expandafter{\romannumeral2} displays a semiconducting state with an energy gap of 0.15 eV, which has an energy gain of 0.42 eV per unit cell compared to the structure-\uppercase\expandafter{\romannumeral1}. The most striking feature of La$_1$phenanthrene is that La $5d$ electrons make a significant contribution to the total density of state (DOS) around the Fermi level, which is distinct from potassium doped phenanthrene and picene. Our findings provide an important foundation for the understanding of superconductivity in La-doped phenanthrene.

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arXiv2013

Numerical analysis of detection-mechanism models of SNSPD

The microscopic mechanism of photon detection in superconducting nanowire single-photon detectors is still under debate. We present a simple, but powerful theoretical model that allows us to identify essential differences between competing detection mechanisms. The model is based on quasi-particle multiplication and diffusion after the absorption of a photon. We then use the calculated spatial and temporal evolution of this quasi-particle cloud to determine detection criteria of three distinct detection mechanisms, based on the formation of a normal conducting spot, the reduction of the effective depairing critical current below the bias current and a vortex-crossing scenario, respectively. All our calculations as well as a comparison to experimental data strongly support the vortex-crossing detection mechanism by which vortices and antivortices enter the superconducting strip from the edges and subsequently traverse it thereby triggering the detectable normal conducting domain. These results may therefore help to reveal the microscopic mechanism responsible for the detection of photons in superconducting nanowires.

cond-mat.supr-con↗

arXiv2013

K and Mn co-doped BaCd2As2: a hexagonal structured bulk diluted magnetic semiconductor with large magnetoresistance

A bulk diluted magnetic semiconductor was found in the K and Mn co-doped BaCd2As2 system. Different from recently reported tetragonal ThCr2Si2-structured II-II-V based(Ba,K)(Zn,Mn)2As2, the Ba1-yKyCd2-xMnxAs2 system has a hexagonal CaAl2Si2-type structure with the Cd2As2 layer forming a honeycomb-like network. The Mn concentration reaches up to its x ? 0.4. Magnetization measurements show that the samples undergo ferromagnetic transitions with Curie temperature up to 16 K. With low coercive field less than 10 Oe and large magnetoresistence of about -70%, the hexagonal structured Ba1-yKyCd2-xMnxAs2 can be served as a promising candidate for spin manipulations.

cond-mat.str-el↗

arXiv2013

Thermal rectification of electrons in hybrid normal metal-superconductor nanojunctions

We theoretically investigate heat transport in hybrid normal metal-superconductor (NS) nanojunctions focusing on the effect of thermal rectification. We show that the heat diode effect in the junction strongly depends on the transmissivity and the nature of the NS contact. Thermal rectification efficiency can reach up to 123% for a fully-transmissive ballistic junction and up to 84% in diffusive NS contacts. Both values exceed the rectification efficiency of a NIS tunnel junction (I stands for an insulator) by a factor close to 5 and 3, respectively. Furthermore, we show that for NS point-contacts with low transmissivity, inversion of the heat diode effect can take place. Our results could prove useful for tailoring heat management at the nanoscale, and for mastering thermal flux propagation in low-temperature caloritronic nanocircuitry.

cond-mat.mes-hall↗

arXiv2013

The electron-phonon relaxation time in thin superconducting titanium nitride films

We report on the direct measurement of the electron-phonon relaxation time, τeph, in disordered TiN films. Measured values of τeph are from 5.5 ns to 88 ns in the 4.2 to 1.7 K temperature range and consistent with a T-3 temperature dependence. The electronic density of states at the Fermi level N0 is estimated from measured material parameters. The presented results confirm that thin TiN films are promising candidate-materials for ultrasensitive superconducting detectors.

cond-mat.supr-con↗

arXiv2014

Surface-resistance measurements using superconducting stripline resonators

We present a method to measure the absolute surface resistance of conductive samples at a set of GHz frequencies with superconducting lead stripline resonators at temperatures 1- 6K. The stripline structure can easily be applied for bulk samples and allows direct calculation of the surface resistance without the requirement of additional calibration measurements or sample reference points. We further describe a correction method to reduce experimental background on high-Q resonance modes by exploiting TEM-properties of the external cabling. We then show applications of this method to the reference materials gold, tantalum, and tin, which include the anomalous skin effect and conventional superconductivity. Furthermore, we extract the complex optical conductivity for an all-lead stripline resonator to find a coherence peak and the superconducting gap of lead.

cond-mat.mes-hall↗

arXiv2014

Magnetic detectability of a finite size paramagnet/superconductor cylindrical cloak

Cloaking of static magnetic fields by a finite thickness type-II superconductor tube surrounded by a coaxial paramagnet shell is studied. On the basis of exact solutions to the London and Maxwell equations, it is shown that perfect cloaking is realizable for arbitrary geometrical parameters including the thin film case for both constituents. In contrast to previous approximate studies assuming perfect diamagnetism of the superconductor constituent, it is proven that cloaking provides simultaneously full undetectability, that is the magnetic moment of the structure completely vanishes as well as all high-order multipole moments as soon as the uniform field outside remains unaffected.

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