Latest Research Papers In Condensed Matter Physics | (Cond-Mat.Mes-Hall) 2019-04-10

Latest Papers in Condensed Matter Physics

Mesoscale And Nanoscale Physics


Thermal conductivity of crystalline AlN and the influence of atomic-scale defects (1904.00345v2)

Runjie Lily Xu, Miguel Munoz Rojo, S. M. Islam, Aditya Sood, Bozo Vareskic, Ankita Katre, Natalio Mingo, Kenneth E. Goodson, Huili Grace Xing, Debdeep Jena, Eric Pop

2019-03-31

Aluminum nitride (AlN) plays a key role in modern power electronics and deep-ultraviolet photonics, where an understanding of its thermal properties is essential. Here we measure the thermal conductivity of crystalline AlN by the 3 method, finding it ranges from 682 56 W/m/K at 100 K to 168 7 W/m/K at 400 K, with a value of 263 6 W/m/K at room temperature. We compare these data with analytical models and first principles calculations, taking into account atomic-scale defects (O, Si, C impurities, and Al vacancies). We find Al vacancies play the greatest role in reducing thermal conductivity because of the largest mass-difference scattering. Modeling also reveals that 50% of heat conduction is contributed by phonons with long mean free paths, over ~0.7 m at room temperature. Consequently, the effective thermal conductivity of AlN is strongly reduced in sub-micron thin films or devices due to phonon-boundary scattering.

Tunable three-way valley Hall energy-splitter: venturing beyond graphene-like structures (1901.01937v3)

Mehul P. Makwana, Gregory J. Chaplain

2019-01-07

Strategically combining four structured domains creates the first ever three-way topological energy-splitter; remarkably, this is only possible using a square, or rectangular, lattice, and not the graphene-like structures more commonly used in valleytronics. To achieve this effect, the two mirror symmetries, present within all fully-symmetric square structures, are broken; this leads to two nondistinct interfaces upon which valley-Hall states reside. These interfaces are related to each other via the time-reversal operator and it is this subtlety that allows us to ignite the third outgoing lead. The geometrical construction of our structured medium allows for the three-way splitter to be adiabatically converted into a wave steerer around sharp bends. Due to the tunability of the energies directionality by geometry, our results have far-reaching implications for applications such as beam-splitters, switches and filters across wave physics.

Ferromagnetic Resonance Studies of Strain tuned Bi:YIG Films (1904.04800v1)

Ravinder Kumar, B. Samantaray, Z. Hossain

2019-04-09

Bismuth-doped Yttrium iron garnet (Bi:YIG) thin films known for large Magneto-optical activity with low losses still needs to get probed for its magnetization dynamics. We demonstrate a controlled tuning of magnetocrystalline anisotropy in Bi-doped Y_3 Fe_5 O_12 (Bi:YIG) films of high crystalline quality using growth induced epitaxial strain on [111]-oriented Gd_3 Ga_5 O_12 (GGG) substrate. We optimize a growth protocol to get thick highly-strained epitaxial films showing large magneto-crystalline anisotropy, compare to thin films prepared using a different protocol. Ferromagnetic resonance measurements establish a linear dependence of the out-of-plane uniaxial anisotropy on the strain induced rhombohedral distortion of Bi:YIG lattice. Interestingly, the enhancement in the magnetoelastic constant due to an optimum substitution of Bi^(3+) ions with strong spin orbit coupling does not strongly affect the precessional damping (~1.15x10^(-3) ). Large magneto-optical activity, reasonably low damping, large magnetocrystalline anisotropy and large magnetoelastic coupling in BiYIG are the properties that may help BiYIG emerge as a possible material for photo-magnonics and other spintronics applications.

Magnetic and electrical transport signatures of uncompensated moments in epitaxial thin films of the non-collinear antiferromagnet MnIr (1904.04797v1)

James M. Taylor, Edouard Lesne, Anastasios Markou, Fasil Kidane Dejene, Pranava Keerthi Sivakumar, Simon Pöllath, Kumari Gaurav Rana, Neeraj Kumar, Chen Luo, Hanjo Ryll, Florin Radu, Florian Kronast, Peter Werner, Christian H. Back, Claudia Felser, Stuart S. P. Parkin

2019-04-09

Non-collinear antiferromagnets, with either an L1 cubic crystal lattice (e.g. MnIr and MnPt) or a D0 hexagonal structure (e.g. MnSn and MnGe), exhibit a number of novel phenomena of interest to topological spintronics. Amongst the cubic systems, for example, tetragonally distorted MnPt exhibits an intrinsic anomalous Hall effect (AHE). However, MnPt only enters a non-collinear magnetic phase close to the stoichiometric composition and at suitably large thicknesses. Therefore, we turn our attention to MnIr, the material of choice for use in exchange bias heterostructures. In this paper, we investigate the magnetic and electrical transport properties of epitaxially grown, face-centered-cubic -MnIr thin films with (111) crystal orientation. Relaxed films of 10 nm thickness exhibit an ordinary Hall effect, with a hole-type carrier concentration of (2.24 0.08) 10 cm. On the other hand, TEM characterization demonstrates that ultrathin 3 nm films grow with significant in-plane tensile strain. This may explain a small remanent moment, observed at low temperatures, shown by XMCD spectroscopy to arise from uncompensated Mn spins. Of the order 0.02 / atom, this dominates electrical transport behavior, leading to a small AHE and negative magnetoresistance. These results are discussed in terms of crystal microstructure and chiral domain behavior, with spatially resolved XML(C)D-PEEM supporting the conclusion that small antiferromagnetic domains, < 20 nm in size, of differing chirality account for the absence of observed Berry curvature driven magnetotransport effects.

Giant violation of Wiedemann-Franz law in doping layers of modern AlGaAs heterostructures (1904.04758v1)

M. Sammon, Mitali Banerjee, B. I. Shklovskii

2019-04-09

We analyze the data of the recent paper Nature 559, 205 (2018) and show that it contains an observation of thermal and electric conductivities of the doping layers in GaAs/AlGaAs heterostructures which violates the Wiedemann-Franz law. Namely, the measured thermal conductivity of the doping layers is similar to that of a metal while the electrical conductivity is exponentially small. We find that these results may be related to the exciton contribution to thermal conductivity calculated in several recent theoretical works for metallic samples.



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