arXiv subjects

G. Koren

Publications and source records attributed to G. Koren.

At least 19 recordsLinked to original sources

arXiv2019

Linear dichroism infrared resonance in over-, under-, and optimally-doped cuprate superconductors

By measuring the polarization changes in THz, infrared, and visible radiation over an extended energy range (3-2330 meV), we observe symmetry-breaking in cuprate high temperature superconductors over wide energy, doping, and temperature ranges. We measure the polarization rotation and ellipticity of transmitted radiation though thin films as the sample is rotated. We observe a two-fold rotational symmetry in the Faraday angle, which is associated with linear dichroism (LD) and occurs when electromagnetic radiation polarized along one direction is absorbed more strongly than radiation polarized in the perpendicular direction. Such polarization anisotropies can be generally associated with symmetry breakings. We measure the amplitude of the LD signal and study its temperature, energy, and doping dependence. The LD signal shows a resonant behavior with a peak in the few hundred meV range, which is coincident with the mid-infrared optical feature that has been associated with the formation of the pseudogap state. The strongest LD signal is found in under-doped films, although it is also observed in optimally- and over-doped samples. The LD signal is consistent with nematic charge ordering as well as novel magnetoelectric effects.

cond-mat.supr-con↗

arXiv2018

Tunneling and bound states in near-edge NbN-$\rm Bi_2Se_3$ junctions fabricated by deposition through a wire shadow mask

Transport measurements in thin film junctions of NbN-$\rm Bi_2Se_3$ exhibit tunneling as well as bound state resonances. The junctions are prepared by pulsed laser deposition of a NbN layer through a 25 $μ$m wide gold wire shadow mask bisecting the wafer into two halves, on a $\rm Bi_2Se_3$ blanket film without further patterning. This results in two independent near-edge junctions connected in series via the 25 $μm$ long and 10 mm wide area of the uncapped $\rm Bi_2Se_3$ layer. Conductance spectra measured across the wire masked trench at different locations on the wafer show that some junctions have tunneling behavior with pronounced coherence peaks at $\pm 2Δ$ where $Δ\simeq$ 1 meV, while others have zero bias conductance peaks and series of bound states at higher bias. The later can be attributed to zero energy Majorana bound states or to the more conventional Andreev bound states. Based on the present results, we can not distinguish between these two scenarios.

cond-mat.supr-con↗

arXiv2013

Optical Birefringence and Dichroism of Cuprate Superconductors in the THz regime

The presence of optical polarization anisotropies, such as Faraday/Kerr effects, linear birefringence, and magnetoelectric birefringence are evidence for broken symmetry states of matter. The recent discovery of a Kerr effect using near-IR light in the pseudogap phase of the cuprates can be regarded as a strong evidence for a spontaneous symmetry breaking and the existence of an anomalous long-range ordered state. In this work we present a high precision study of the polarimetry properties of the cuprates in the THz regime. While no Faraday effect was found in this frequency range to the limits of our experimental uncertainty (1.3 milli-radian or 0.07$^\circ$), a small but significant polarization rotation was detected that derives from an anomalous linear dichroism. In YBa$_2$Cu$_3$O$_y$ the effect has a temperature onset that mirrors the pseudogap temperature T$^*$ and is enhanced in magnitude in underdoped samples. In $x=1/8$ La$_{2-x}$Ba$_{x}$CuO$_4$, the effect onsets above room temperature, but shows a dramatic enhancement near a temperature scale known to be associated with spin and charge ordered states. These features are consistent with a loss of both C$_4$ rotation and mirror symmetry in the electronic structure of the CuO$_2$ planes in the pseudogap state.

cond-mat.str-el↗

arXiv2011

Proximity induced superconductivity by Bi in topological $Bi_2Te_2Se$ and $Bi_2Se_3$ films: Evidence for a robust zero energy bound state possibly due to Majorana Fermions

Point contact conductance measurements on topological $Bi_2Te_2Se$ and $Bi_2Se_3$ films reveal a signature of superconductivity below 2-3 K. In particular, critical current dips and a robust zero bias conductance peak are observed. The latter suggests the presence of zero energy bound states which could be assigned to Majorana Fermions in an unconventional topological superconductor. We attribute these novel observations to proximity induced local superconductivity in the films by small amounts of superconducting Bi inclusions or segregation to the surface, and provide supportive evidence for these effects.

cond-mat.supr-con↗

arXiv2011

Photoinduced melting of superconductivity in the high-Tc superconductor La2-xSrxCuO4 probed by time-resolved optical and THz techniques

Dynamics of depletion and recovery of superconducting state in La2-xSrxCuO_4 thin films is investigated utilizing optical pump-probe and optical pump - THz probe techniques as a function of temperature and excitation fluence. The absorbed energy density required to suppress superconductivity is found to be about 8 times higher than the thermodynamically determined condensation energy density and nearly temperature independent between 4 and 25 K. These findings indicate that during the time when superconducting state suppression takes place (~0.7 ps), a large part (nearly 90%) of the energy is transferred to the phonons with energy lower than twice the maximum value of of the SC gap and only 10% is spent on Cooper pair breaking.

cond-mat.supr-con↗

arXiv2010

Interface effects in d-wave superconductor-ferromagnet junctions

Measurements of the differential conductance spectra of YBa2Cu3O7-SrRuO3 and YBa2Cu3O7-La0.67Ca_0.33MnO3 ramp-type junctions along the node and anti-node directions are reported. The results are consistent with a crossed Andreev reflection effect only in YBa2Cu3O7-SrRuO3 junctions where the domain wall width of SrRuO3 is comparable with the coherence length of YBa2Cu3O7. No such effect was observed in the YBa2Cu3O7-La0.67Ca0.33MnO3 junctions, which is in line with the much larger (x10) domain wall width of La0.67Ca0.33MnO3. We also show that crossed Andreev exists only in the anti-node direction. Furthermore, we find evidence that crossed Andreev in YBa2Cu3O7 junctions is not sensitive to nm-scale interface defects, suggesting that the length scale of the crossed Andreev effect is larger than the coherence length, but still smaller than the La0.67Ca0.33MnO3's domain wall width.

cond-mat.supr-con↗

arXiv2009

Conventional proximity effect in bilayers of superconducting underdoped $La_{1.88}Sr_{0.12}CuO_4$ islands coated with non superconducting overdoped $La_{1.65}Sr_{0.35}CuO_4$

Following a recent study by our group in which a large $T_c$ enhancement was reported in bilayers of the non-superconducting $La_{1.65}Sr_{0.35}CuO_4$ and superconducting $La_{1.88}Sr_{0.12}CuO_4$ films [Phys. Rev. Lett. \textbf{101}, 057005 (2008)], we checked if a similar effect occurs when superconducting $La_{1.88}Sr_{0.12}CuO_4$ islands are coated with a continuous layer of the non superconducting $La_{1.65}Sr_{0.35}CuO_4$. We found that no such phenomenon is observed. The bare superconducting islands film behaves as an insulator where transport occurs via hopping or tunneling between islands, but exhibits a weak signature of localized superconductivity at $\sim$18 K. When over coated with a $La_{1.65}Sr_{0.35}CuO_4$ film, it becomes superconducting with a $T_c$ onset of 16.6 K, which is less than that of a thick $La_{1.88}Sr_{0.12}CuO_4$ film (25.7 K). We therefore conclude that the lower $T_c$ in the bilayer is due to a conventional proximity effect.

cond-mat.supr-con↗

arXiv2008

Evidence for pairing above Tc from the dispersion in the pseudogap phase of cuprates

In the underdoped high temperature superconductors, instead of a complete Fermi surface above Tc, only disconnected Fermi arcs appear, separated by regions that still exhibit an energy gap. We show that in this pseudogap phase, the energy-momentum relation of electronic excitations near E_F behaves like the dispersion of a normal metal on the Fermi arcs, but like that of a superconductor in the gapped regions. We argue that this dichotomy in the dispersion is hard to reconcile with a competing order parameter, but is consistent with pairing without condensation.

cond-mat.supr-con↗

arXiv2007

Quantum vortex tunneling in $YBa_2Cu_3O_{7-δ}$ thin films

Cuprate films offer a unique opportunity to observe vortex tunneling effects, due to their unusually low superfluid density and short coherence length. Here, we measure the magnetoresistance (\textit{MR}) due to vortex motion of a long meander line of a superconducting film made of underdoped $YBa_2Cu_3O_{7-δ}$. At low temperatures (\textit{T}), the \textit{MR} shows a significant deviation from Arrhenius activation. The data is consistent with two dimensional Variable Range Hopping (VRH) of single vortices, i.e. $MR\propto exp[-(T_0/T)^{1/3}]$. The VRH temperature scale $T_0$ depends on the vortex tunneling rates between pinning sites. We discuss its magnitude with respect to estimated parameters of the meander thin film.

cond-mat.supr-con↗

arXiv2007

A study of the ferromagnetic transition of $SrRuO_3$ in nanometer thick bilayers with $YBa_2Cu_3O_y$, $La_{1.88}Sr_{0.12}CuO_{4-y}$, Au and Cr: Signature of injected carriers in the pseudogap regime

The hypothesis regarding the existence of uncorrelated pre-formed pairs in the pseudogap regime of superconducting $YBa_2Cu_3O_y$ is tested experimentally using bilayers of $YBa_2Cu_3O_y$ and the itinerant ferromagnet $SrRuO_3$. In our study, we monitor the influence of $YBa_2Cu_3O_y$ on $T_p$, the ferromagnetic ordering temperature of $SrRuO_3$. Here, $T_p$ is the temperature of maximum dM/dT or dR/dT where M and R are the magnetization and resistance of $SrRuO_3$, respectively. We compare the results with similar measurements carried out on bilayers of $La_{1.88}Sr_{0.12}CuO_{4-y}$, $Au$ and $Cr$ with $SrRuO_3$. We find that in bilayers made of underdoped 10 nm $YBa_2Cu_3O_y$/5 nm $SrRuO_3$, the $T_p$ values are shifted to lower temperatures by up to 6-8 K as compared to $T_p\approx 140$ K of the 5 nm thick reference $SrRuO_3$ film. In contrast, in the other type of bilayers, which are not in the pseudogap regime near $T_p$, only a smaller shift of up to $\pm$2 K is observed. These differences are discussed in terms of a proximity effect, where carriers from the $YBa_2Cu_3O_y$ layer are injected into the $SrRuO_3$ layer and vice versa. We suggest that correlated electrons in the pseudogap regime of $YBa_2Cu_3O_y$ are responsible for the observed large $T_p$ shifts.

cond-mat.supr-con↗

arXiv2006

Flux-flow resistivity anisotropy in the instability regime in the a-b plane of epitaxial YBCO thin films

Measurements of the nonlinear flux-flow resistivity $ρ$ and the critical vortex velocity $\rm v^*_ϕ$ at high voltage bias close to the instability regime predicted by Larkin and Ovchinnikov \cite{LO} are reported along the node and antinode directions of the d-wave order parameter in the \textit{a-b} plane of epitaxial $YBa_2Cu_3O_{7-δ}$ films. In this pinning-free regime, $ρ$ and $\rm v^*_ϕ$ are found to be anisotropic with values in the node direction larger on average by 10% than in the antinode direction. The anisotropy of $ρ$ is almost independent of temperature and field. We attribute the observed results to the anisotropic quasiparticle distribution on the Fermi surface of $YBa_2Cu_3O_{7-δ}$.

cond-mat.supr-con↗

arXiv2005

Signature of a crossed Andreev reflection effect (CARE) in the magnetic response of YBCO junctions with the itinerant ferromagnet SrRuO_3

Magnetic properties of SFS and SF ramp-type junctions with $YBa_2Cu_3O_{7-δ}$ (YBCO) electrodes (S), and the itinerant ferromagnet $SrRuO_3$ (SRO - F), were investigated. We looked for a crossed Andreev reflection effect (CARE) in which an electron from one magnetic domain in F is Andreev reflected as a hole into an adjacent, oppositely polarized, domain while a pair is transmitted into S. CARE is possible in SRO since the width of its domain walls is of the order of the YBCO coherence length (2-3 nm). Our junctions behave as typical magnetic tunneling junctions, as the conductance spectra were always asymmetric, and a few showed bound state peaks at finite bias that shifted with field according to the classical Tedrow and Meservey theory. In many of our SFS junctions with a barrier thickness of 10-20 nm, a prominent zero bias conductance peak (ZBCP) has been observed. This peak was found to decrease \textit{linearly} with magnetic field, as expected for Andreev and CARE scattering. In contrast, in SF junctions, the observed ZBCP was found to decrease versus field almost \textit{exponentially}, in agreement with the Anderson-Appelbaum theory of scattering by magnetic states in F. Thus, transport in our SFS and SF junctions depends strongly on the size of the F layer. We also found that in both cases, the ZBCP height at zero field decreased with increasing magnetic order of the domains in F, in agreement with the CARE mechanism.

cond-mat.supr-con↗

arXiv2005

Experimental search for anisotropic flux flow resistivity in the a-b plane of optimally doped epitaxial thin films of YBCO

Transport measurements along the node and anti-node directions in the a-b plane of optimally doped and epitaxial thin films of YBCO are reported. Low bias magnetoresistance measurements near and below T_c show that the flux flow resistivity along the node and anti-node directions versus magnetic field are indistinguishable. This result suggests that within the experimental error of our measurements, no correspondence is found between the flux pinning properties in YBCO and the d-wave nature of the order parameter.

cond-mat.supr-con↗

arXiv2005

Magnetoresistance of Junctions made of Underdoped YBCO Separated by a Ga-doped YBCO Barrier

We report magnetoresistance measurements of ramp type superconductor-normal-superconductor (SNS) junctions. The junctions consist of underdoped $YBa_{2}Cu_{3}O_y$ (YBCO) electrodes separated by a barrier of $YBa_{2}Cu_{2.6}Ga_{0.4}O_y$. We observe a large positive magnetoresistance, linear in the field. We suggest that this unusual magnetoresistance originates in the field dependence of the proximity effect. Our results indicate that in underdoped YBCO/N/YBCO SNS structures, the proximity effect does not exhibit the anomalously long range found in optimally doped YBCO structures. From our data we obtain the diffusion coefficient and relaxation time of quasiparticles in underdoped YBCO.

cond-mat.supr-con↗

arXiv2005

Anisotropic magnetization of RuEu1.5Ce0.5Sr2Cu2O10 (Ru-1222) thin films

A dc magnetic study is reported on c-axis oriented epitaxial thin films of EuSr2Eu1.5Ce0.5Cu2O10 (Ru-1222) on (100) SrTiO3 wafers. Magnetic measurements on 200 and 500 nm thick films were performed with the magnetic field either parallel or perpendicular to the wafer. We found that the films order magnetically at 180 K. The easy axis of the magnetization is in the basal plane and the Ru5+ ions are in their high-spin state, as deduced from the isothermal magnetization curves at low temperatures. The anisotropy ratio was found to be 7.5. The observed results are compared to data of the Ru-1222 ceramic material.

cond-mat.supr-con↗

arXiv2004

Observation of a New Mechanism of Spontaneous Generation of Magnetic Flux in a Superconductor

We report the discovery of a new mechanism of spontaneous generation of a magnetic flux in a superconductor cooled through $T_c$. The sign of the spontaneous flux changes randomly from one cooldown to the next, and follows a Gaussian distribution. The width of the distribution increases with the size of the temperature gradient in the sample. Our observations appear inconsistent with the well known mechanisms of flux generation. The dependence on the temperature gradient suggests that the flux may be generated through an instability of the thermoelectric superconducting-normal quasiparticle counterflow.

cond-mat.supr-con↗

arXiv2003

Observation of McMillan-Rowell like oscillations in underdoped YBCO junctions oriented along the node of the d-wave order parameter

Dynamic resistance spectra of ramp junctions made of underdoped YBCO electrodes and Ga-doped YBCO barrier are reported. Series of equidistant peaks were observed in these spectra in junctions oriented along the node direction. Junctions with different barrier thickness d_N, showed that the distance between adjacent peaks scales inversely with d_N. The peaks were thus identified as due to McMillan-Rowell like oscillations in the barrier. Analysis of the series of peaks yields an upper limit of about 3.7 meV on the value of the energy gap along the node. We attribute this small gap to the complex s component of the order parameter of underdoped YBCO near the interface of the junctions.

cond-mat.supr-con↗

arXiv2003

Observation of a common symmetry for the pseudogap and the superconducting order parameter near the surface of underdoped YBCO

Measurements of the angular dependence of conductance spectra in the a-b plane of underdoped YBCO junctions are reported. At zero magnetic field the superconducting gap shows a |d+is|-like symmetry. Application of a magnetic field strongly suppresses this gap leaving only the pseudogap feature which also shows a |d+is|-like angular dependence. We thus observe the same symmetry for the superconducting gap and the pseudogap characterizing the YBCO electrodes near the interface with the barrier. An H_{c2} value of about 5T of the secondary (is) order parameter can also be deduced from our results.

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