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Fingerprints of Majorana modes beyond the zero-bias conductance peak

Satoshi Ikegaya

The unambiguous detection of Majorana bound states (MBSs) in topological superconductors has been a central topic of condensed matter physics for recent years. So far, the presence of MBSs was demonstrated experimentally in a number of topologically nontrivial superconducting systems. In this connection, clear evidences of Majorana bound states are only obtained by the detection of zero-bias conductance peaks in tunneling transport measurements. In recent years, it became clear that various additional signatures of Majorana modes need to be investigated in order to complete our understanding.
In our presentation, we summarize two unambiguous fingerprints which can act as a ‘smoking gun’evidence. First we study the anomalous nonlocal conductance due to chiral Majorana edge states in a superconductor/ferromagnet hybrid as shown in Fig. 1(a). We obtain the important result that the chiral nature of the Majorana edge states causes an anomalously long-range and chirality-sensitive nonlocal transport in this device. This, in turn, enables us to identify conclusively the moving direction and further properties of the chiral Majorana edge states [1]. Secondly, we propose a novel experiment for achieving the first experimental observation of the anomalous proximity effect caused by Majorana bound states. In particular, we discuss the differential conductance of a semiconductor/superconductor hybrid as shown in Fig. 1(b), which contains a planar topological Josephson junction realized in recent experiments. The conductance spectrum changes drastically through the topological phase transition because the Majorana bound state appearing only in the topologically nontrivial phase can penetrate into the dirty normal segment and form the resonant transmission channel there [2]. In general, our results allow contrasting singlet and triplet superconductivity employing properties of Majorana modes beyond zero bias peaks.

[1] S. Ikegaya, Y. Asano, and D. Manske, Phys. Rev. Lett. 123, 207002 (2019)
[2] S. Ikegaya, S. Tamura, D. Manske, and Y. Tanaka, arXiv: 2007.12888 (2020)

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Odd-frequency pairing in topological superconductors

Yukio Tanaka

It is known that odd-frequency pairing ubiquitously presents in superconductor junctions [1]. Especially, in the presence of zero energy surface Andreev bound state (ZESABS) realized in topological superconductors, the odd-frequency pairing is amplified near the surface or interface. One of the remarkable property generated by odd-frequency pairing is anomalous proximity effect in diffusive normal metal (DN) / superconductor junction where quasiparticle density of states in DN has a zero energy peak (ZEP) of LDOS due to the penetration of odd-frequency spin-triplet s-wave pairing [2,3]. It has been shown that proximity coupled nano-wire junction [4] is an idealistic system to study anomalous proximity effect due to odd-frequency triplet-s wave pairing [5].
We have further clarified the relation between induced odd-frequency pairing and the bulk quantity defined by Green’s function[6]. Odd-frequency Cooper pairs with chiral symmetry emerging at the edges are a useful physical quantity. We have shown that the odd-frequency Cooper pair amplitudes can be expressed by a winding number extended to a nonzero frequency and can be evaluated from the spectral features of the bulk. We have found that the odd-frequency Cooper pair amplitudes are classified into two categories: the amplitudes in the first category have the singular functional form proportional to 1/z (where z is a complex frequency) that reflects the presence of ZESABS, whereas the amplitudes in the second category have the regular form proportional to z.
Recently, we have found that the presence of ZESABS generates new type of thermopower.
We have shown that the thermoelectric effect in ferromagnet / superconductor junctions can be entirely dominated by ingap Andreev reflection processes. Consequently, the electric current from a temperature bias changes sign in the presence of ZESABS and resulting odd-frequency pairing [7].

[1]Y. Tanaka, M. Sato and N. Nagaosa, J. Phys. Soc. Jpn. 81 011013 (2012)
[2]Y. Tanaka and A.A. Golubov, Phys. Rev. Lett. 98 037003 (2007)
[3]Y. Tanaka, A.A. Golubov, S. Kashiwaya, and M. Ueda, Phys. Rev. Lett. 99 037005 (2007); Y. Tanaka, Y. Tanuma, and A. A. Golubov, Phys. Rev. B 96 054552 (2007)
[4]Y. Tanaka and S. Kashiwaya, Phys. Rev. B 70 012507 (2004)
[5]R. M. Lutchyn, J. D. Sau, and S. Das Sarma, Phys. Rev. Lett. 105, 077001 (2010), Y. Oreg, G. Refael, and F. von Oppen, Phys. Rev. Lett. 105, 177002 (2010)
[6]Y. Asano and Y. Tanaka, Phys. Rev. B 87 104513 (2013)
[7]S. Ta mura, S. Hoshino and Y. Tanaka, Phys. Rev. B 99 , 184512 (2019)
[8]T. Savander , S. Tamura, C. Flindt, Y. Tanaka and P. Burset , arXiv: 2008.00849

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Propagation and interference of d-wave superconducting pairs in graphene

Javier Villegas

Following earlier work in which we demonstrated the Klein-like tunneling of d-wave superconducting paints intro graphene [1], here we present experiments that show the longrange preparation of d-wave correlations in that material. For this, we fabricated devices that behave as a proximitized Fabry-Pérot cavitiy, where d-wave Andreev pairs interferences are produced. The interferences manifest themselves in series of pronounced conductance oscillations analogous to those produced by De Gennes-Saint James resonances in conventional superconductor/metal junctions. Their observation imply that the d-wave Andreev pairs propagate over distances of a few hundred nm in the CVD graphene used for the experiments [2]. We will end up by discussing ongoing experiments in applied magnetic field, which also produces an intriguing series of conductance oscillations in the superconducting state of the junctions.

[1] D. Perconte, F. A. Cuellar, C. Moreau-luchaire, M. Piquemal-Banci, R. Galceran, P. R. Kidambi, M.-B. Martin, S. Hofmann, R. Bernard, B. Dlubak, P. Seneor, and J. E. Villegas, Nat. Phys. 14, 25 (2018)
[2] D. Perconte, K. Seurre, V. Humbert, C. Ulysse, A. Sander, J. Trastoy, V. Zatko, F. Godel, P. R. Kidambi, S. Hofmann, X. P. Zhang, D. Bercioux, F. S. Bergeret, B. Dlubak, P. Seneor, and J. E. Villegas, Phys. Rev. Lett. 125, 87002 (2020)

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Poster Session

 

Poster 13 Mátyás Kocsis Department of Physics, BUTE Tuning the nonreciprocal resistance of BiTeBr
Poster 14 Kinga Lasek University of South Florida Molecular Beam Epitaxy of Self-Intercalated Transition Metal Tellurides
Poster 15 Soo Yeon Lim Sogang University Thickness dependent magnetic transition of MnPS3
Poster 16 Mingzu Liu The Pennsylvania State University Tunable RT FM in 1L V-WS2 & V-WSe2 via CVD
Poster 17 Francisco Munoz Universidad de Chile Magnon Hall Effect in CrI3-based vdW systems
Poster 18 Tianxiao Nie Beihang University 2D ferromagnetic materials above room temperature
Poster 19 Sergey Nikolaev Tokyo Institute of Technology Realistic modelling of monolayer NbS2 and NbSe2
Poster 20 Armando Pezo Federal University of ABC TMDC/Graphene an ab initio study
Poster 21 Charis Quay UniversitÈ Paris-Saclay Tunneling spectroscopy of few-monolayer NbSe2
Poster 22 Akhil Rajan University of St Andrews Morphology control of monolayer transition metal dichalcogenides by MBE
Poster 23 Patrick Reiser University of Basel Scanning NV Magnetometry of 2D Magnetism

 

Intrinsic 2D-XY ferromagnetism in a van der Waals monolayer

Amilcar Bedoya-Pinto

Long before the recent fascination with two-dimensional materials, the critical behaviour and universality scaling of phase transitions in low-dimensional systems has been a topic of great interest. Recent experiments on layered magnetic systems show that a sizable out-of-plane magnetic anisotropy is able to stabilize 2D long-range ferromagnetic order, as demonstrated in CrI3, CrBr3, Fe3GeTe2 and Cr2Ge2Te6 [1], while a spontaneous magnetic ordering has remained elusive for an in-plane 2D magnetic system in the monolayer limit. Here, we construct a nearly ideal easy-plane system, a CrCl3 monolayer grown on Graphene/6H-SiC (0001), which exhibits ferromagnetic ordering as unambiguously determined by element-specific X-ray magnetic dichroism [2]. Hysteretic behaviour of the field-dependent magnetization is sustained up to a temperature of 10 K, and angular dependent measurements evidence a clear in-plane easy axis, unlike all other van der Waals monolayer magnets reported to date. The origin of the easy-plane anisotropy is discussed in terms of a non-zero orbital moment and a trigonal distortion of the CrCl3 unit cell. Moreover, the analysis of the critical exponents of the temperature-dependent magnetization show a scaling behaviour that is characteristic of a 2D-XY system. These observations suggest the first realization of a finite-size Berezinskii-Kosterlitz-Thouless (BKT) phase transition in a quasi-freestanding monolayer magnet with a XY universality class; accessible through the bottom-up growth of a van der Waals layer with an in-plane hexagonal crystal symmetry and negligible substrate interaction.

Figure 1. (a) Schematic crystal structure of CrCl3/Graphene/6H-SiC layers in top view and cross-section configurations. (b) Atom resolved image of the CrCl3 lattice featuring a moiré pattern, which corresponds to a 23.8° rotation between the hexagonal unit cell of CrCl3 and graphene.(c) XMCD hysteresis loops taken in grazing (in-plane) and normal (out-of-plane) incidence, evidencing a weak anisotropy favouring an in-plane easy axis. (d) Modified Arrott-Plots for the temperature-and field dependent XMCD data. A consistent set of critical exponents is inferred (β=0.235, γ= 2.2), matching with the predictions of the 2DXY model.

References
[1] K. S. Burch, D. Mandrus, J. G. Park, Nature. 563, 47–52 (2018)
[2] A. Bedoya-Pinto et al., arXiV https://arxiv.org/abs/2006.07605 (2020)

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New Developments on Chromium Trihalides 2D Ferromagnets

David Soriano

The discovery of 2D ferromagnets in 2017 has opened new ways to explore novel physical phenomena at the nanoscale. In the first part of my talk, I will briefly discuss the recent advances in chromium trihalides, from fundamentals to applications.[1] In the second part, I will focus on our recent work regarding the role of Coulomb interactions in the intralayer exchange, the electrical tunability of interlayer magnetism in bilayer CrI3,[2] and the exchange proximity effects in van der Waals heterostructures containing chromium trihalides.[3,4] [1] Magnetic Two-Dimensional Chromium Trihalides: A Theoretical Perspective. D. Soriano, M. I. Katsnelson, and J. Fernández-Rossier. Submitted to Nano Letters.

[2] Magnetic polaron and antiferromagnetic-ferromagnetic transition in doped bilayer CrI3. D. Soriano, and M. I. Katsnelson. Phys. Rev. B 101, 041402(R) (2020)

[3] Van der Waals Spin Valves. C. Cardoso, N. A. García-Martínez, and J. Fernández-Rossier. Phys. Rev. Lett. 121, 067701 (2018)

[4] Exchange-bias controlled correlations in magnetically encapsulated twisted van der Waals dichalcogenides. D. Soriano, and J. L. Lado. arXiv:2006.09953

On-line SPICE-SPIN+X Seminars

On-line Seminar: 8 July 2020 - 15:00 (CET)

Towards deep neural networks with nanoscale spintronic oscillators as neurons

Julie Grollier, CNRS-Thales

Spintronic oscillators are nanoscale devices realized with magnetic tunnel junctions which have the potential to be integrated by hundreds of millions in electronic chips. Their non-linear dynamical properties are rich and tunable, and can be leveraged to imitate different features of biological neurons. High performance pattern recognition was achieved through the coupled dynamics of the oscillators in small circuits. The transient dynamics of a single spintronic nano-oscillator has been used to implement reservoir computing, achieving state-of-the-art results on a simple spoken digit recognition task [1], [2]. Four spintronic nano-oscillators have been trained to classify spoken vowels by phase locking their oscillations to the strong input signals produced by external microwave sources [3]. Three spintronic nano-oscillators did bind temporal data through their mutual synchronization [4].

These demonstrations now need to be scaled to deep networks to establish their potential definitely. The neocortex, the seat of higher cognitive functions in the brain, has a hierarchical structure of six layers of neurons. Adopting such a layered structure in artificial neural networks was the key to their fantastic progress in the last ten years. Neuromorphic systems need to be scalable to deep networks to truly establish their promises.

PDF file of the talk available here

A key asset of spintronic nano-oscillators towards this goal is their ability to emit radio-frequency (RF) signals. These oscillators indeed produce microwave voltages with varying amplitude and frequency in response to direct current inputs. They could therefore potentially communicate through radio-frequencies signals, allowing fully parallel operation with minimized wiring, at a speed seven orders of magnitude faster than the brain. But for this, it is necessary to devise radio-frequency synapses that can interconnect the oscillators.

In this talk, I will rapidly review recent results on neuromorphic computing with spintronic nano-oscillators. I will then describe how they can be interconnected layer-wise through RF spintronic nano-synapses, and present our recent simulation results of classification with these novel RF synapses.

[1]  J. Torrejon et al., « Neuromorphic computing with nanoscale spintronic oscillators », Nature, 547,  428‑431 (2017).
[2]   S. Tsunegi et al., « Physical reservoir computing based on spin torque oscillator with forced synchronization », Appl. Phys. Lett., 114,  164101 (2019).
[3]   M. Romera et al., « Vowel recognition with four coupled spin-torque nano-oscillators », Nature, 563, 230,(2018).
[4]   M. Romera et al., « Binding events through the mutual synchronization of spintronic nano-neurons », arXiv:2001.08044 (2020).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 1 July 2020 - 15:00 (CET)

Magnons as Probes of Strongly Correlated Electron Physics

Amir Yacoby, Harvard University

Scattering experiments have revolutionized our understanding of nature. Examples include the discovery of the nucleus, crystallography, and the discovery of the double helix structure of DNA. Scattering techniques differ by the type of the particles used, the interaction these particles have with target materials and the range of wavelengths used. Here, we demonstrate a new 2-dimensional table-top scattering platform for exploring magnetic properties of materials on mesoscopic length scales. Long lived, coherent magnonic excitations are generated in a thin film of YIG and scattered off a magnetic target deposited on its surface. The scattered waves are then recorded using a scanning NV center magnetometer that allows sub-wavelength imaging and operation under conditions ranging from cryogenic to ambient environment. While most scattering platforms measure only the intensity of the scattered waves, our imaging method allows for spatial determination of both amplitude and phase of the scattered waves thereby allowing for a systematic reconstruction of the target scattering potential. Our experimental results are consistent with theoretical predictions for such a geometry and reveal several unusual features of the magnetic response of the target, including suppression near the target edges and gradient in the direction perpendicular to the direction of surface wave propagation. Our results establish magnon scattering experiments as a new platform for studying correlated many-body systems.

PDF file of the talk available here

 

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 24 June 2020 - 15:00 (CET)

Chiral spintronics: non collinear spin textures with application to Racetrack Memory

Stuart Parkin, Max Planck Institute of Microstructure Physics and
Martin Luther University Halle-Wittenberg, Halle

Magnetic non-collinear spin textures that have chiral structures are of great current interest.  Coupled chiral domain walls in synthetic antiferromagnetic (SAF) racetracks and compensated ferrimagnets can be moved with current at very high speeds via a novel giant exchange torque1,2. When the antiferromagnetic coupling between two domain walls in a SAF racetrack is weakened unusual magnetization dynamics takes place due to chiral domain wall drag3. The same type of Dzyaloshinskii-Moriya (DMI) vector exchange interactions that stabilize chiral Néel domain walls in magnetic multilayers results in the formation of topological spin textures in bulk compounds. We recently discovered magnetic antiskyrmions in a tetragonal inverse Heusler compound Mn1.4Pt0.9Pd0.1Sn using Lorentz transmission electron microscopy (LTEM) 4. The size of the anti-skyrmion can be tuned by varying the thickness of the host material allowing for sizes varying from nanometer to microns in the same material5.  This is due to long range magneto-dipole interactions that are important in this compound that has a complex DMI interaction with a symmetry that follows the D2d symmetry of its crystalline structure. The same symmetry ensures that anti-skyrmions are robust to temperature and magnetic field6. The magnetic dipole-dipole interactions also allow for the formation of metastable “elliptical skyrmions” in this same material7.  These have Bloch-like boundaries that we can directly observe using LTEM. Finally, we discuss our recent discovery of novel Néel like skyrmions in a metallic compound that exist almost to room temperature and that are also tunable in size8.  Chiral spin textures in ferro-, ferri- and anti-ferrimagnetic materials and thin film heterostructures are of fundamental interest with potential for spintronic applications.

PDF file of the talk available here

 

References

1  Yang, S.-H., Ryu, K.-S. & Parkin, S. S. P. Domain-wall velocities of up to 750 ms−1 driven by exchange-coupling torque in synthetic antiferromagnets. Nat. Nano. 10, 221-226, (2015).
2  Bläsing, R. et al. Exchange coupling torque in ferrimagnetic Co/Gd bilayer maximized near angular momentum compensation temperature. Nat. Commun. 9, 4984, (2018).
3  Yang, S.-H., Garg, C. & Parkin, S. Chiral Exchange Drag and Chirality Oscillations in Synthetic Antiferromagnets Nat. Phys. 15, 543–548, (2019).
4  Nayak, A. K. et al. Magnetic antiskyrmions above room temperature in tetragonal Heusler materials. Nature 548, 561-566, (2017).
5  Ma, T. et al. Tunable Magnetic Antiskyrmion Size and Helical Period from Nanometers to Micrometers in a D2d Heusler Compound. Adv. Mater., 2002043, (2020).
6  Saha, R. et al. Intrinsic stability of magnetic anti-skyrmions in the tetragonal inverse Heusler compound Mn1.4Pt0.9Pd0.1Sn Nat. Commun. 10, 5305, (2019).
7  Jena, J. et al. Elliptical Bloch skyrmion chiral twins in an antiskyrmion system. Nat. Commun. 11, 1115, (2020).
8  Srivastava, A. K. et al. Observation of Robust Néel Skyrmions in Metallic PtMnGa. Adv. Mater. 32, 1904327, (2020).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 17 June 2020 - 15:00 (CET)

Coherent Sub-Terahertz Spin Pumping from an Insulating Antiferromagnet

Enrique del Barco, University of Central Florida

Emerging phenomena, such as the spin-Hall effect (SHE), spin pumping, and spin-transfer torque (STT), allow for interconversion between charge and spin currents and the generation of magnetization dynamics that could potentially lead to faster, denser, and more energy efficient, non-volatile memory and logic devices. Present STT-based devices rely on ferromagnetic (FM) materials as their active constituents. However, the flexibility offered by the intrinsic net magnetization and anisotropy for detecting and manipulating the magnetic state of ferromagnets also translates into limitations in terms of density (neighboring elements can couple through stray fields), speed (frequencies are limited to the GHz range), and frequency tunability (external magnetic fields needed). A new direction in the field of spintronics is to employ antiferromagnetic (AF) materials. In contrast to ferromagnets, where magnetic anisotropy dominates spin dynamics, in antiferromagnets spin dynamics are governed by the interatomic exchange interaction energies, which are orders of magnitude larger than the magnetic anisotropy energy, leading to the potential for ultrafast information processing and communication in the THz frequency range, with broadband frequency tunability without the need of external magnetic fields.

PDF file of the talk available here

I will present evidence of sub-terahertz coherent spin pumping at the interface of a uniaxial insulating antiferromagnet MnF2 and platinum thin films, measured by the ISHE voltage signal arising from spin-charge conversion in the platinum layer. The ISHE signal depends on the chirality of the dynamical modes of the antiferromagnet, which is selectively excited and modulated by the handedness of the circularly polarized sub-THz irradiation (see figure). Contrary to the case of ferromagnets, antiferromagnetic spin pumping exhibits a sign dependence on the chirality of dynamical modes, allowing for the unambiguous distinction between coherent spin pumping and the thermally-driven, chirality-independent spin Seebeck effect. Our results open the door to the controlled generation of coherent pure spin currents with antiferromagnets at unprecedented high frequencies.

This work has been primarily supported by the Air Force Office of Scientific Research under Grant FA9550-19-1-0307.

References

  • Priyanka Vaidya, Sophie A. Morley, Johan van Tol, Yan Liu, Ran Cheng, Arne Brataas, David Lederman, and Enrique del Barco, Subterahertz spin pumping from an insulating antiferromagnet, Science 368, 160-165 (2020)

 

 

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