arXiv subjects

Y. Dagan

Publications and source records attributed to Y. Dagan.

At least 37 records · Page 2Linked to original sources

arXiv2014

Anomalous magnetic ground state in LaAlO3/SrTiO3 interface probed by transport through nanowires

Resistance as a function of temperature down to 20mK and magnetic fields up to 18T for various carrier concentrations is measured for nanowires made from the SrTiO3/LaAlO3 interface using a hard mask shadow deposition technique. The narrow width of the wires (of the order of 50nm) allows us to separate out the magnetic effects from the dominant superconducting ones at low magnetic fields. At this regime hysteresis loops are observed along with the superconducting transition. From our data analysis we find that the magnetic order probed by the giant magnetoresistance (GMR) effect vanishes at TCurie = 954 mK. This order is not a simple ferromagnetic state but consists of domains with opposite magnetization having a preferred in-plane orientation.

cond-mat.mes-hall↗

arXiv2014

One-dimensional Quantum Wire Formed at the Boundary Between Two Insulating LaAlO3/SrTiO3 Interfaces

We grow a tiled structure of insulating two dimensional LaAlO3/SrTiO3 interfaces composed of alternating one and three LaAlO3 unit cells. The boundary between two tiles is conducting. At low temperatures this conductance exhibits quantized steps as a function of gate voltage indicative of a one dimensional channel. The step size of half the quantum of conductance is an evidence for absence of spin degeneracy.

cond-mat.mes-hall↗

arXiv2014

Phase Diagram of Micron-Size Bridges of SrTiO$_3/$LaAlO$_3$ Interface: Link Between Multiple Band Structure and Superconductivity

The rich phase diagram of the two dimensional electron gas (2DEG) at the \STO/\LAO interface is probed using Hall and longitudinal resistivity. Thanks to a special bridge design we are able to tune through the superconducting transition temperature T$_c$ and to mute superconductivity by either adding or removing carriers in a gate bias range of a few volts. Hall signal measurements pinpoint the onset of population of a second mobile band right at the carrier concentration where maximum superconducting T$_c$ and critical field H$_c$ occur. These results emphasize the advantages of our design, which can be applied to many other two dimensional systems assembled on top of a dielectric substrate with high permittivity.

cond-mat.supr-con↗

arXiv2014

Tuning the critical temperature of cuprate superconductor films using self-assembled organic layers

Many of the electronic properties of high-temperature cuprate superconductors (HTSC) are strongly dependent on the number of charge carriers put into the CuO$_2$ planes (doping). Superconductivity appears over a dome-shaped region of the doping-temperature phase diagram. The highest critical temperature (Tc) is obtained for the so-called "optimum doping". The doping mechanism is usually chemical; it can be done by cationic substitution. This is the case, for example, in La$_{2-x}$Sr$_x$CuO$_4$ where La3+ is replaced by Sr2+ thus adding a hole to the CuO$_2$ planes. A similar effect is achieved by adding oxygen as in the case of YBa$_2$Cu$_3$O$_{6+δ}$ where $δ$ represents the excess oxygen in the sample. In this paper we report on a different mechanism, one that enables the addition or removal of carriers from the surface of the HTSC. This method utilizes a self-assembled monolayer (SAM) of polar molecules adsorbed on the cuprate surface. In the case of optically active molecules, the polarity of the SAM can be modulated by shining light on the coated surface. This results in a light-induced modulation of the superconducting phase transition of the sample. The ability to control the superconducting transition temperature with the use of SAMs makes these surfaces practical for various devices such as switches and detectors based on high-Tc superconductors.

cond-mat.supr-con↗

arXiv2013

Evolution of the fermi surface of a doped topological insulator with carrier concentration

In an ideal bulk topological-insulator (TI) conducting surface states protected by time reversal symmetry enfold an insulating crystal. However, the archetypical TI, Bi2Se3, is actually never insulating; it is in fact a relatively good metal. Nevertheless, it is the most studied system among all the TIs, mainly due to its simple band-structure and large spin-orbit gap. Recently it was shown that copper intercalated Bi2Se3 becomes superconducting and it was suggested as a realization of a topological superconductor (TSC). Here we use a combination of techniques that are sensitive to the shape of the Fermi surface (FS): the Shubnikov-de Haas (SdH) effect and angle resolved photoemission spectroscopy (ARPES) to study the evolution of the FS shape with carrier concentration, n. We find that as n increases, the FS becomes 2D-like. These results are of crucial importance for understanding the superconducting properties of CuxBi2Se3.

cond-mat.mes-hall↗

arXiv2012

Magnetotransport effects in polar versus non-polar SrTiO3 based heterostructures

Anisotropic magnetoresistance and negative magnetoresistance for in-plane fields are compared for the LaAlO3 /SrTiO3 interface and the symmetric Nb-doped SrTiO3 heterostructure. Both effects are exceptionally strong in LaAlO3 /SrTiO3 . We analyze their temperature, magnetic field and gate voltage dependencies and find them to arise from a Rashba type spin-orbit coupling with magnetic scatterers that have two contributions to their potential: spin exchange and Coulomb interaction. Atomic spin-orbit coupling is sufficient to explain the small effects observed in Nb-doped SrTiO3 . These results clarify contradicting transport interpretations in SrTiO3 -based heterostructures.

cond-mat.str-el↗

arXiv2012

Probing the surface states in Bi2Se3 using the Shubnikov-de Haas effect

Shubnikov-de Haas (SdH) oscillations are observed in Bi2Se3 flakes with high carrier concentration and low bulk mobility. These oscillations probe the protected surface states and enable us to extract their carrier concentration, effective mass and Dingle temperature. The Fermi momentum obtained is in agreement with angle resolved photoemission spectroscopy (ARPES) measurements performed on crystals from the same batch. We study the behavior of the Berry phase as a function of magnetic field. The standard theoretical considerations fail to explain the observed behavior.

cond-mat.mes-hall↗

arXiv2011

Low-temperature dependence of the thermo-magnetic transport properties of the SrTiO3/LaAlO3 interface

We report transport measurements, including: Hall, Seebeck and Nernst Effect. All these transport properties exhibit anomalous field and temperature dependences, with a change of behavior observed at about H 1.5T and T 15K. We were able to reconcile the low-temperature-low-field behavior of all transport properties using a simple two band analysis. A more detailed model is required in order to explain the high magnetic field regime.

cond-mat.str-el↗

arXiv2011

Anomalous Enhancement of the Superconducting Transition Temperature in Electron-Doped Cuprate Heterostructures

The superconducting transition temperature $T_{c}$ of multilayers of electron-doped cuprates, composed of underdoped (or undoped) and overdoped La% $_{2-x}$Ce$_{x}$CuO$_{4}$ (LCCO) and Pr$_{2-x}$Ce$_{x}$CuO$_{4}$ (PCCO) thin films, is found to increase significantly with respect to the $T_{c}$ of the corresponding single-phase films. By investigating the critical current density of superlattices with different doping levels and layer thicknesses, we find that the $T_{c}$ enhancement is caused by a redistribution of charge over an anomalously large distance.

cond-mat.supr-con↗

arXiv2011

Anomalous magneto-transport at the superconducting interface between LaAlO3 and SrTiO3

The magnetoresistance as a function of temperature and field for atomically flat interfaces between 8 unit cells of LaAlO3 and SrTiO3 is reported. Anomalous anisotropic behavior of the magnetoresistance is observed below 30 K for superconducting samples with carrier concentration of 3.5\times10^13 cm^-2 . We associate this behavior to a magnetic order formed at the interface.

cond-mat.str-el↗

arXiv2010

Phase coherent transport in SrTiO3/LaAlO3 interfaces

The two dimensional electron gas formed between the two band insulators SrTiO3 and LaAlO3 exhibits a variety of interesting physical properties which make it an appealing material for use in future spintronics and/or quantum computing devices. For this kind of applications electrons have to retain their phase memory for sufficiently long times or length. Using a mesoscopic size device we were able to extract the phase coherence length, and its temperature variation. We find the dephasing rate to have a power law dependence on temperature. The power depends on the temperature range studied and sheet resistance as expected from dephasing due to strong electron-electron interactions.

cond-mat.mes-hall↗

arXiv2010

Shubnikov-de Haas oscillations in SrTiO3\LaAlO3 interface

Quantum magnetic oscillations in SrTiO3/\LaAlO3 interface are observed. The evolution of their frequency and amplitude at various gate voltages and temperatures is studied. The data are consistent with the Shubnikov de-Haas theory. The Hall resistivity rho exhibits nonlinearity at low magnetic field. It is fitted assuming multiple carrier contributions. The comparison between the mobile carrier density inferred from the Hall data and the oscillation frequency suggests multiple valley and spin degeneracy. The small amplitude of the oscillations is discussed in the framework of the multiple band scenario.

cond-mat.str-el↗

arXiv2010

On the resistivity at low temperatures in electron-doped cuprate superconductors

We measured the magnetoresistance as a function of temperature down to 20mK and magnetic field for a set of underdoped PrCeCuO (x=0.12) thin films with controlled oxygen content. This allows us to access the edge of the superconducting dome on the underdoped side. The sheet resistance increases with increasing oxygen content whereas the superconducting transition temperature is steadily decreasing down to zero. Upon applying various magnetic fields to suppress superconductivity we found that the sheet resistance increases when the temperature is lowered. It saturates at very low temperatures. These results, along with the magnetoresistance, cannot be described in the context of zero temperature two dimensional superconductor-to-insulator transition nor as a simple Kondo effect due to scattering off spins in the copper-oxide planes. We conjecture that due to the proximity to an antiferromagnetic phase magnetic droplets are induced. This results in negative magnetoresistance and in an upturn in the resistivity.

cond-mat.supr-con↗

arXiv2010

Femtosecond Quasiparticle and Phonon Dynamics in Superconducting YBa2Cu3O7 Studied by Wideband Terahertz Spectroscopy

We measure the anisotropic mid-infrared response of electrons and phonons in bulk YBa2Cu3O7 after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150 fs demonstrating that the lattice absorbs a major portion of the pump energy before the quasiparticles are thermalized. Our results attest to substantial electron-phonon scattering and introduce a powerful concept probing electron-lattice interactions in a variety of complex materials.

cond-mat.supr-con↗

arXiv2010

Tuning spin-orbit coupling and superconductivity at the SrTiO3/LaAlO3 interface: a magneto-transport study

The superconducting transition temperature, Tc, of the SrTiO3/LaAlO3 interface was varied by the electric field effect. The anisotropy of the upper critical field and the normal state magneto-transport were studied as a function of gate voltage. The spin-orbit coupling energy is extracted. This tunable energy scale is used to explain the strong gate dependence of the mobility and of the anomalous Hall signal observed. The spin-orbit coupling energy follows Tc for the electric field range under study.

cond-mat.str-el↗

arXiv2009

Anisotropic magneto-transport effects at SrTiO3\LaAlO3 interfaces

The resistivity as a function of temperature, magnetic field and its orientation for atomically flat SrTiO3\LaAlO3 interfaces with carrier densities of ~3*10^13 cm^-2 is reported. At low magnetic fields superconductivity is observed below 130mK. The temperature dependence of the high field magnetoresistance and its strong anisotropy suggest possible magnetic ordering below 35K. The origin of this ordering and its possible relation to superconductivity are discussed.

cond-mat.str-el↗

arXiv2009

Single superconducting energy scale in electron-doped cuprate superconductor Pr_2-xCexCuO_4

The tunneling spectra of the electron-doped cuprate Pr_2-xCe_xCuO4 as a function of doping and temperature is reported. We find that the superconducting gap, delta, shows a BCS-like temperature dependence even for extremely low carrier concentrations (studied here for the first time). Moreover, delta follows the doping dependence of Tc, in strong contrast with tunneling studies of hole-doped cuprates. From our results we conclude that there is a single superconducting energy scale in the electron-doped cuprates.

cond-mat.supr-con↗

arXiv2008

Field Induced Nodal Order Parameter in the Tunneling Spectrum of YBa$_2$Cu$_3$O$_{7-x}$ Superconductor

We report planar tunneling measurements on thin films of YBa$_2$Cu$_3$O$_{7-x}$ at various doping levels under magnetic fields. By choosing a special setup configuration, we have probed a field induced energy scale that dominates in the vicinity of a node of the d-wave superconducting order parameter. We found a high doping sensitivity for this energy scale. At Optimum doping this energy scale is in agreement with an induced $id_{xy}$ order parameter. We found that it can be followed down to low fields at optimum doping, but not away from it.

cond-mat.supr-con↗