Latest Papers in Condensed Matter Physics

Mesoscale And Nanoscale Physics

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Selective and Fast Plasmon-Assisted Photo-Heating of Nanomagnets; A New Route for Opto-Activated Nanomagnetic Logic and Artificial Spin Systems (1902.04021v1)

Matteo Pancaldi, Naëmi Leo, Paolo Vavassori

2019-02-11

Thermal relaxation of nanoscale magnetic islands, mimicking Ising macrospins, is indispensable for studies of geometrically frustrated artificial spin systems and low-energy nanomagnetic computation. Currently-used heating schemes based on contact to a thermal reservoir, however, lack the speed and spatial selectivity required for the implementation in technological applications. Applying a hybrid approach by combining a plasmonic nanoheater with a magnetic element, in this work we establish the robust and reliable control of local temperatures in nanomagnetic arrays by contactless optical means. Plasmon-assisted photo-heating allows for temperature increases of up to several hundred Kelvins, which lead to thermally-activated moment reversals and a pronounced reduction of the magnetic coercive field. Furthermore, the polarization-dependent absorption cross section of elongated plasmonic elements enables sublattice-specific heating on sub-nanosecond time scales. Using optical degrees of freedom, i.e. focal position, polarization, power, and pulse length, thermoplasmonic heating of nanomagnets offers itself for the use in flexible, fast, spatially-, and element-selective thermalization for functional magnetic metamaterials.

Strong intravalley scattering on graphene corrugations revealed by Raman spectroscopy (1801.08861v2)

Péter Kun, Gergő Kukucska, Gergely Dobrik, János Koltai, Jenő Kürti, László P. Biró, Levente Tapasztó, Péter Nemes-Incze

2018-01-26

The pseudo-magnetic field generated by mechanical strain in graphene can have dramatic consequences on the behavior of electrons and holes. Here we show that pseudo-magnetic field fluctuations present in crumpled graphene can induce significant intravalley scattering of charge carriers. We detect this by measuring the confocal Raman spectra of crumpled areas, where we observe an increase of the D'/D peak intensity ratio by up to a factor of 300. We reproduce our observations by numerical calculation of the double resonant Raman spectra and interpret the results as experimental evidence of the phase shift suffered by Dirac charge carriers in the presence of a pseudo-magnetic field. This lifts the restriction on complete intravalley backscattering of Dirac fermions.

Energy spectrum of two-dimensional excitons in a non-uniform dielectric medium (1902.03962v1)

M. R. Molas, A. O. Slobodeniuk, K. Nogajewski, M. Bartos, Ł. Bala, A. Babiński, K. Watanabe, T. Taniguchi, C. Faugeras, M. Potemski

2019-02-11

We demonstrate that, in monolayers (MLs) of semiconducting transition metal dichalcogenides, the -type Rydberg series of excitonic states follows a simple energy ladder: , =1,2,\ldots, in which is the Rydberg energy scaled by the dielectric constant of the medium surrounding the ML and by the reduced effective electron-hole mass, whereas the ML polarizability is only accounted for by . This is justified by the analysis of experimental data on excitonic resonances, as extracted from magneto-optical measurements of a high-quality WSe ML encapsulated in hexagonal boron nitride (hBN), and well reproduced with an analytically solvable Schr"odinger equation when setting the electron-hole potential in the form of a modified Kratzer potential. Applying our convention to other, MoSe, WS, MoS MLs encapsulated in hBN, we estimate an apparent magnitude of for each of the studied structures. Intriguingly, is found to be close to zero for WSe as well as for MoS monolayers, what implies that the energy ladder of excitonic states in these two-dimensional structures resembles that of Rydberg states of a three-dimensional hydrogen atom.

Magnetism and Magneto-optical Effects in Bulk and Few-layer CrI: A Theoretical GGA + U Study (1902.03944v1)

Vijay Kumar Gudelli, Guang-Yu Guo

2019-02-11

The latest discovery of ferromagnetism in atomically thin films of semiconductors CrGeTe and CrI has unleashed numerous opportunities for fundamental physics of magnetism in two-dimensional (2D) limit and also for technological applications based on 2D magnetic materials. In this paper, we present a comprehensive theoretical study of the magnetic, electronic, optical and magneto-optical(MO) properties of multilayers [monolayer(ML), bilayer and trilayer] and bulk CrI, based on the density functional theory with the generalized gradient approximation plus on-site Coulomb repulsion scheme. Interestingly, all the structures are found to be single-spin ferromagnetic(FM) semiconductors. They all have a large out-of-plane magnetic anisotropy energy(MAE) of 0.5 meV/Cr. These large MAEs suppress transverse spin fluctuations and thus stabilize long-range magnetic orders at finite temperatures down to the ML limit. They also exhibit strong MO effects with their Kerr and Faraday rotation angles being comparable to that of best-known bulk MO materials. The shape and position of the main features in the optical and MO spectra are found to be nearly thickness-independent although the magnitude of Kerr rotation angles increases monotonically with the film thickness. Magnetic transition temperatures estimated based on calculated exchange coupling parameters, calculated optical conductivity, MO Kerr and Faraday rotation angles agree quite well with available experimental data. The calculated MAE as well as optical and MO properties are analyzed in terms of the calculated orbital-decomposed densities of states, band state symmetries and dipole selection rules. Our findings of large out-of-plane MAEs and strong MO effects in these single-spin FM semiconducting CrI ultrathin films suggest that they will find valuable applications in semiconductor MO and spintronic nanodevices.

Counter-directional polariton coupler (1902.03933v1)

M. Klaas, J. Beierlein, E. Rozas, S. Klembt, H. Suchomel, T. H. Harder, K. Winkler, M. Emmerling, H. Flayac, M. D. Martín, L. Viña, S. Höfling, C. Schneider

2019-02-11

We report on an on-chip routing device for propagating condensates of exciton-polaritons. This counterdirectional coupler implements signal control by a photonic microdisk potential, which couples two lithographically defined waveguides and reverses the condensate's propagation direction. By varying the structural sizes, we utilize the conjunction of the different dimensionalities to additionally evidence the functionality of a polaritonic resonant tunnel diode. Furthermore, we investigate the ultra fast dynamics of the device via ps-resolved streak camera measurements, which is distinctive for the polariton platform. This scalable, all-directional coupler element is a central building block for compact non-linear on-chip photonic architectures.

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