High-Tc cuprates | story of two electronic subsystems

Prof. Neven Barišić, Viena University of Technology, Austria and University of Zagreb, Croatia

19th November 2019 at 11:15, Seminar room for physics

Abstract

The high-Tc cuprates are amongst the most intensively studied correlated materials. Nevertheless, pivotal questions regarding their principal phases and regimes, as well as the transitions between them, remain unanswered. This is, largely, due to the complexity of these materials that renders the extraction of intrinsic properties difficult. We have performed a thorough experimental study of HgBa2CuO4+δ, which, in many respects, is a model cuprate compound. From the comparison of our measurements with data for other cuprates, we are able to separate universal behavior from compound-specific features. This exercise leads to a series of remarkable findings, the most important of which are that the effective mass and the scattering rate remain essentially unchanged across the phase diagram [1,2], and that the scattering rate is dominated by an umklapp process [3]. These novel insights enabled an accurate count of charges across the phase diagram. The electronic system is thus found to consist of 1+p charges, where p corresponds to doping. At low dopings, within the pseudogap regime, exactly one hole is localized per planar copper-oxygen unit. Upon increasing doping and temperature, the hole is gradually delocalized and becomes itinerant [4]. The overall behaviors are consistent with a gradual extension of Fermi arcs to a full Fermi surface, without an essential change of the underlying Fermi surface that encloses 1+p states. Finally, we have established that the itinerant Fermi-liquid holes become superconducting while the localized hole provides the glue.

[1] N. Barišić et al, New J. Phys. 21, 113007 (2019)
[2] Li et al, Phys. Rev. Lett. 117, 197001 (2016)
[3] N. Barišić et al, Proc. Natl. Acad. Sci. 110, 12235 (2013); S. I. Mirzaei et al, Proc.
Natl. Acad. Sci. 110, 5774 (2013); M. Chan et al, Phys. Rev. Lett. 113, 177005 (2014); P.
Popčević et al, Quantum Mater. 3, 42 (2018)
[4] D. Pelc et al, Sci. Adv. 5, eaau4538 (2019)

Abstract_Barisic

The evolution order parameters in an auxetic Liquid Crystal Elastomers under strain

Thomas Raistrick
School of Physics and Astronomy, University of Leeds, UK

Abstract:

Liquid crystal elastomers combine the fields of liquid crystal physics with polymer physics with surprising consequences. Our group has reported a negative order parameter of the polymer backbone of an LCE based on stress strain curves. This coincides with a negative Poisson ratio and occurs during a reorientation of the nematic director.
The evolution of order parameters, P2 and P4, for an LCE under strain has been determined using Raman spectroscopy. This revealed that, for our LCE, P4>P2 for certain values of strain. A behaviour not seen in conventional nematic thermotropic liquid crystals. The region where P4>P2 coincides with the beginning of the mechanical Freedrickz transition. Subsequent experiments on the LCE prepared in different states to understand the deformation modes. These are reported herein.

Poster

Tunable optical diffraction gratings based on a ferromagnetic liquid crystal

Mathias Fleisch, Faculty of Physics, University of Vienna, Austria

 Abstract:

The director of a ferromagnetic liquid crystal can be realigned by small external magnetic fields. Transmission optical diffraction gratings composed of periodic slices of a ferromagnetic liquid crystal and a conventional photoresist polymer are demonstrated. Dependence of diffraction efficiencies of various diffraction orders on an in-plane external magnetic field is investigated. It is shown that diffraction properties can be effectively tuned by magnetic fields as low as a few mT. The experimental results are explained  with analytical calculations based on the Jones calculus for polarising microscope measurements and simulations using the Rigorous Coupled Wave Analysis (RCWA) for the diffraction experiments.

References:

S. Gao, M. Fleisch, R. A. Rupp, L. Cmok, P. Medle-Rupnik, A. Mertelj, D. Lisjak, X. Zhang, and I. Drevenšek-Olenik, “Magnetically tunable optical diffraction gratings based on a ferromagnetic liquid crystal,” Opt. Express 27 (2019)

Poster

Electrostatic interaction between magnetic nanoplatelets in alcohols

Patricija Hribar Boštjančič, Department of Complex Matter, Jožef Stefan Institute

Abstract:

In a room–temperature liquid magnet barium hexaferrite (BHF) nanoplatelets are spontaneously ordered and form a ferromagnetic nematic phase. In concentrated suspension in 1-butanol the nanoplatelets with magnetic moments perpendicular to their basal plane align in large macroscopic regions, forming magnetic domains. The key parameter for the suspension stability and the formation of the ferromagnetic nematic phase are electrostatic interactions, which can be influenced by the solvent and the concentration of surfactant, i.e., dodecylbenzenesulfonic acid (DBSA). We investigated the parameters that affect the electrostatic interaction between the BHF nanoplatelets in alcohol suspensions. The measurements of electrophoretic mobility and conductivity were performed in tert-butanol, 1-hexanol, 1-butanol and 2-propanol suspensions to consider also the effect of solvent’s dielectric constant. I will present our results of the alcohol’s dielectric constant effect on the equilibrium between dissolved and adsorbed DBSA and consequently on the electrostatic interaction and the Debye screening length.

Poster [PDF]

Fabrication of thin film transistors and energy storage devices from liquid phase exfoliated nanosheets

Dr. Victor Vega-Mayoral, CRANN & AMBER research centers, Trinity College Dublin, School of physics, Trinity College Dublin

Abstract:

Transition metal dichalcogenides has been one of the most promising family of materials for a lustrum. Strong light matter interaction and high mobilities when exfoliated down to the monolayer. Liquid phase exfoliation produces dispersions of few-layer TMDCs. Due to the liquid nature of the final sample it is possible to print or spray these dispersions to produce thin films of 2d flakes. We will talk about the fabrication of transistors and energy storage devices.

TiS2 is a promising material for energy storage devices. Its application has been reduced due to a fast degradation in open atmosphere. Our latest results proofs how degradation can be avoided by carefully choosing the solvent. We have fabricated stable batteries with high cyclability and capacities close to the theoretical value.

Mixed networks of conducting and non-conducting nanoparticles show promise in a range of applications where fast charge transport is important. While the dependence of network conductivity on the conductive mass fraction (Mf) is well understood, little is known about the Mf-dependence of mobility and carrier density. I  will show here a detailed characterization of thin film transistors as a function of the WS2-graphene Mf. Here, we use electrolytic gating to investigate the transport properties of spray-coated composite networks of graphene and WS2 nanosheets.

Poster [PDF]

SrₓBi₂Se₃-nematicity, superconductivity, crystals and thin films

Aleksander Yu. Kuntsevich, P. N. Lebedev Physical Institute, Moscow, Russia

Abstract:

3D topological insulator Bi₂Se₃ attracts much attention as a platform for future low consumption spintronics and quantum computations. Recently, nematic (and possibly topological) superconductivity was discovered in AₓBi₂Se₃, where A = Cu, Sr, Nb. In my talk I will discuss phenomenology of the nematicity, observed in SrₓBi₂Se₃ single crystals in both superconducting and normal states. I will also review our efforts on molecular beam epitaxy growth of both parent compound Bi₂Se₃ and Sr-doped Bi₂Se₃ thin films. The latter appear to be non-superconducting because Sr atoms in the films get different positions than in the crystals. Our results call for novel growth approaches for design of superconducting SrₓBi₂Se₃ thin films.

Poster [PDF]

Transient cooling of quasiparticles in K3C60  by mid-infrared laser pulses 

 Prof. Dr. Michele Fabrizio, International School for Advanced Studies SISSA, Trieste, Italy

Abstract:

We propose an explanation of the transient superconducting-like optical behavior observed at temperatures as high as ten times Tc in K₃C₆₀ irradiated by mid-infrared laser pulses.  In our theory the phenomenon is due to the laser pulse effectively cooling down low energy quasiparticles much below the external temperature. The mechanism is quite general and relies on the existence of  localised excitations, in K₃C₆₀ these are spin-triplet excitons,  that act as entropy sink when the laser is on, while, when the laser is off, they release back the stolen entropy very gradually.

Poster [PDF]

Magnetotransport properties of 2DEG formed in LAO/ETO/STO heterostructures studied using the electric field effect

Maria D’Antuono, University of Naples Federico II, Naples, Italy

Abstract:

In 2004 Ohtomo and Hwang reported the formation of a high mobility 2-dimensional electron gas (2DEG) at the interface between two wide bandgap insulators oxides, namely LaAlO₃ (LAO) and SrTiO₃ (STO). In this work, we show that the 2DEG created at the LAO/STO interface becomes both electric-field-tunable spin polarized and superconducting by introducing a few atomic layers of EuTiO₃ (ETO) which is an antiferromagnetic (AF) insulator iso-structural to STO . Among the most interesting characteristics of this 2DEG are the remarkably large Rashba-spin-orbit interactions and unconventional superconductivity and magnetism, possibly related to the presence of strong correlations in quantum-confined 3d-bands. The occurrence of magnetic interactions, superconductivity and spin-orbit interactions in the same 2DEG system makes the LAO/ETO/STO an intriguing platform for the emergence of novel quantum phases in low-dimensional devices. The main goal of this work was to investigate the electrical transport of the LAO/ETO/STO interface and to shed more light on the complex nature of this system and its phase diagram. In particular, we focused on the interplay between Rashba-spin-orbit interactions and ferromagnetism. In fact, the LAO/ETO/STO 2DEG is one of the few systems where such interplay can be studied and it is therefore of great interest for future spintronic applications.

Poster [PDF]

Spectrally resolved and three pulse pump-probe spectroscopy in strongly correlated organic conductors
Dr. Satoshi Tsuchiya, Complex Matter Department, Jozef Stefan Institute, Slovenia; Department of Applied Physics, Hokkaido University, Japan

Abstract:

The series of k-(BEDT-TTF)2X (X: anion molecules) has attracted much interest in fundamental physics related with strong electron correlation and application of devices because of a rich variety of electronic phases under equilibrium such as the Mott insulator and superconductor, as well as nonequilibrium related with photoinduced phase transition. In my long-term stay in JSI, spectrally-resolved pump probe spectroscopy was carried out to investigate superconducting phase in this system. On the other hand, three-pulse pump probe spectroscopy was applied for photoinduced phase separation. In the seminar, I will show the obtained results briefly and discuss in detail.

Poster [PDF]

High-Tc superconductivity in ruthenates
Asst. Prof. Dr. Hiroyoshi Nobukane, Department of Physics, Hokkaido University, Sapporo, Japan

Abstract:

Layered perovskite materials can control quantum states from superconductivity to a Mott insulator by tuning physical and chemical pressure. By reducing the thickness of a layered crystal to nanometer range, a nanofilm due to a negative pressure effect leads to novel quantum states that have not been found yet in bulk crystals. We will discuss the emergence of high-Tc superconductivity near 100 K in Ca₂RuO₄ nanofilms. A thin film of Ca₂RuO₄ exhibits supercurrent and typical Berezinskii-Kosterlitz-Thouless transition behavior. We also found that the tuned  film thickness and the induced bias current cause a superconductor-insulator transition. Our results demonstrate the presence of two-dimensional superconductivity from a high temperature. The fabrication of nanofilms made of layered material enables us to discuss rich superconducting phenomena in ruthenates.

Poster [PDF]

Out of equilibrium electronic properties of ZrTe5
Michele Diego, Department of Complex Matter, JSI

Abstract:

Transition metal pentatelluride, ZrTe₅, displays a set of unique and exotic transport properties, which make this material an ideal candidate for magnetic and thermoelectric devices. In particular, the best known anomaly is its resistivity trend, which shows a metallic behavior or a semiconductor behavior,  respectively, for temperatures below or above a critical temperature. Several theoretical models have been proposed to interpret this anomaly, based on charge density wave or polaron formation, but with no direct experimental verification. In this work we report on the temperature dependence of the ZrTe₅ valence band, studied at equilibrium and out of equilibrium, by means of time and angle resolved photoemission spectroscopy. Our results unveil the dependence of the band structure across the Fermi level on the sample temperature. By performing temperature dependent experiments, we are able to observe an energy shift of the ZrTe₅ valence band. The same effect is observed both by varying the equilibrium sample temperature and by optically exciting the system out-of equilibrium.
Finally, by following the out-of-equilibrium time evolution of the valance band, we report the relaxation times of its position, width and intensity.

Poster [PDF]

Nonresonant multi-pulse selective spectroscopy of interaction-induced responses in liquids using optical Kerr effect

Kazan E.K. Zavoisky Physical-Technical Institute, Kazan, Russia

Abstract:

An ultrafast selective spectroscopic measurements of vibrational-rotational dynamics of molecules in liquids were implemented using optical Kerr effect (OKE) detection under multi-pulse nonresonant xcitation. Introducing additional excitation pulses and creating either constructive or destructive interference of corresponding wave packets allows one to isolate the responses of certain molecular motions (orientational,inter-orintramolecular)in OKE signal. As a result,the  parameters of molecular rotations and coherent vibrations could be evaluated with significantly greater precision compared to conventional single pulse excitation scenario.

Nanostructured superconducting single-photon detectors as photon energy, number, and polarization resolving devices

Departments of Electrical and Computer Engineering and Physics and Astronomy, and the Materials Science Program and Laboratory for Laser Energetics, University of Rochester, Rochester, NY

Abstract:

We present an overview of the physics of operation of superconducting single-photon detectors (SSPDs) and their implementation as the photon-energy, photon-number, and polarization resolving devices. The detection mechanism of SSPDs is based on photon-induced hotspot formation and, subsequent, generation of a voltage transient across a nanostructured superconducting NbN meander (~4-nm-thick and ~100-nm-wide stripe). The NbN SSPD operates in the 4.2-2 K temperature range. The best devices exhibit quantum efficiency of up to near 100% (when encapsulated in a cavity) in the near-infrared (1550 nm) wavelength range, dark count rates <1 Hz, and the noise-equivalent power (NEP) of ~510-21 W/Hz.1/2 For our photon-energy resolution studies, we have adopted a statistical method based on a well-documented fact that quantum efficiency (QE) of SSPDs very strongly (quasi-exponentially) depends on the photon wavelength and the normalized current bias. Thus, by measuring the SSPD QE at different bias levels, we were able to resolve the wavelength of the incident photons with a 50-nm resolution. In another approach, we have implemented a low-noise, cryogenic high-electron-mobility transistor (HEMT) as a very high impedance element to separate the 50-Ω output transmission line from the SSPD. This arrangement allowed us to achieve some amplitude resolution of the recorded output transients and, subsequently, photons with different energies could be distinguished by comparing the output transient amplitude distributions. Next, by designing SSPDs with different physical geometries, we could unambiguously demonstrate their sensitivity to photon polarization. At the end of our presentation, we will present new directions of the SSPD research, focusing on superconductor/ferromagnetic nanostripes.

Photoinduced carrier dynamics in organic superconducting system: Previous results and recent progress

Dr. Satoshi Tsuchiya
Department of Applied Physics, Hokkaido University, Hokkaido, Japan

Ultrafast spin density wave dynamics at intense optical pulse excitation

Mimoza Naseska
Fakulteta za matematiko in fiziko in Institut »Jožef Stefan«, Ljubljana

Abstract:

Control of Light Field Based on Liquid Crystal/Polymer Composite Structures

prof. dr. Xinzheng Zhang
TEDA Institute of Applied Physics & School of Physics, Nankai University, P. R. China

Abstract:

Nanometric thin organic and inorganic layers and their use in perovskite solar cells and elastomers

Peter Topolovšek
Jožef Stefan International Postgraduate School, Ljubljana

Invitation and abstract [PDF]

Abstract:

Realization of highly performing vertically built opto-electronic devices typically requires a close control over the thickness of the constituing layers in order to find the balance between the conformity and optical and electronic properties of the layers. Specifically in solar cells, charge selective layers should be thick enough to allow pinhole free coverage and thin enough to minimize the electrical series resistance and the parasitic absorption. In the talk I will first present recently developed applications of thin organic and inorganic layers in perovskite solar cells which provide conformal coverage and minimization of material consumption while retaining selectivity of the charge transport over nanometric distances. Furthermore I will briefly describe how the 2D nature of PbI2 can be exploited for environmentally friendly synthesis of MAPbI3 submicron sized particles and how the surface modification of chemically exfoliated MoS2 can promote its homogeneous incorporation into elastomers.

Graphene-related materials research @ Joanneum research – Materials

dr. Reinhard Kaindl
Joanneum Research, Graz, Austria
Invitation [PDF]

Photophysics of transition metal dichalcogenides obtained from liquid phase exfoliation

Victor Vega Mayoral
Trinity College Dublin, Dublin, Ireland

Abstract:

Two research lines are presented in this seminar. The first one is focused on exfoliating WS2 via liquid phase exfoliation and the use of some basic, commonly available and fast characterization techniques. Exfoliation in water/polymer dispersions has been studied. Poly(vynil alcohol) (PVA) is proposed as a model polymeric stabilizer. Effects of polymer concentration and centrifugation size- selection protocol on concentration, thickness and lateral size are reported. Extinction, Raman and photoluminescence spectroscopy were used for a fast and semiquantitative characterization supported by the more conventional TEM and AFM microscopy. The second research line focuses on a deeper characterization of the photoexcited states dynamics of few-layer MoS2 and WS2 where we have used spectrally-resolved femtosecond pump–probe spectroscopy. Photoexcited states dynamics in few-layer WS2 have beeen studied using femtosecond pump-probe spectroscopy. Using a cascade model I obtained an exciton dissociation lifetime of 1.3 ps. After excitons dissociation the geminated charges diffuse till they are trapped in defects such as grain boundaries or sulphur vacancies. There is a longer relaxation process that lasts hundreds of picoseconds assigned to charge recombination.

Photophysics of atomically thin MoS2 devices

Daniele Vella
Jožef Stefan International Postgraduate School, Ljubljana