<center><b>Latest Papers in Condensed Matter Physics</b></center>

# <center>Mesoscale And Nanoscale Physics</center> 
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### [Non-Hermitian Boundary Modes](http://arxiv.org/abs/1902.07217v2) (1902.07217v2)
<b>Dan S. Borgnia, Alex Jura Kruchkov, Robert-Jan Slager</b>

<i>2019-02-19</i>
> We consider conditions for the existence of boundary modes in non-Hermitian systems with edges of arbitrary co-dimension. Through a universal formulation of formation criteria for boundary modes in terms of local Green functions, we outline a generic perspective on the appearance of such modes and generate corresponding dispersion relations. In the process, we explain the skin effect in both topological and non-topological systems, exhaustively generalizing bulk-boundary correspondence in the presence of non-Hermiticity. This is accomplished via a doubled Green's function, inspired by doubled Hamiltonian methods used to classify Floquet and, more recently, non-Hermitian topological phases. Our work constitutes a general tool, as well as, a unifying perspective for this rapidly evolving field. Indeed, as a concrete application we find that our method can expose novel non-Hermitian topological regimes beyond the reach of previous methods.

### [Hyperfine level structure in nitrogen-vacancy centers near the  ground-state level anticrossing](http://arxiv.org/abs/1805.01251v2) (1805.01251v2)
<b>Marcis Auzinsh, Andris Berzins, Dmitry Budker, Laima Busaite, Ruvin Ferber, Florian Gahbauer, Reinis Lazda, Arne Wickenbrock, Huijie Zheng</b>

<i>2018-05-03</i>
> Energy levels of nitrogen-vacancy centers in diamond were investigated using optically detected magnetic-resonance spectroscopy near the electronic ground-state level anticrossing (GSLAC) at an axial magnetic field around 102.4~mT in a diamond sample with a nitrogen concentration of 1~ppm. By applying radiowaves in the frequency ranges from 0 to 40 MHz and from 5.6 to 5.9 GHz, we observed transitions that involve energy levels mixed by the hyperfine interaction. We developed a theoretical model that describes the level mixing, transition energies, and transition strengths between the ground-state sublevels, including the coupling to the nuclear spin of the NV center\textquotesingle s ![](http://latex2png.com/output//latex_eab8cef730f7394199c22c704820da6e.png)N and ![](http://latex2png.com/output//latex_5874242198c9a54f3d6e102f3e3c3510.png)C atoms. The calculations were combined with the experimental results by fitting the ODMR spectral lines based on a theoretical model, which yielded information about the polarization of nuclear spins. This study is important for the optimization of experimental conditions in GSLAC-based applications, e.g., microwave-free magnetometry and microwave-free nuclear-magnetic-resonance probes.

### [Topological thermal Hall effect of "magnetic monopoles" in pyrochlore  U(1) spin liquid](http://arxiv.org/abs/1904.08865v1) (1904.08865v1)
<b>Xiao-Tian Zhang, Yong Hao Gao, Chunxiao Liu, Gang Chen</b>

<i>2019-04-18</i>
> "Magnetic monopole" is an exotic quantum excitation in pyrochlore U(1) spin liquid, and its emergence is purely of quantum origin and has no classical analogue. We predict topological thermal Hall effect (TTHE) of "magnetic monopoles" and present this prediction through non-Kramers doublets. We observe that, when the external magnetic field polarizes the Ising component of the local moment, internally this corresponds to the induction of emergent dual U(1) gauge flux for the "magnetic monopoles". The motion of "magnetic monopoles" is then twisted by the induced dual gauge flux. This emergent Lorentz force on "magnetic monopoles" is the fundamental origin of TTHE. Therefore, TTHE would be a direct evidence of the "monopole"-gauge coupling and the emergent U(1) gauge structure in pyrochlore U(1) spin liquid. Our result does not depend strongly on our choice of non-Kramers doublets for our presentation, and can be well extended to Kramers doublets. Our prediction can be readily tested among the pyrochlore spin liquid candidate materials. We give a detailed discussion about the expectation for different pyrochlore magnets.

### [Electrical detection of current generated spin in topological insulator  surface states: Role of interface resistance](http://arxiv.org/abs/1810.00757v2) (1810.00757v2)
<b>Connie H. Li, Olaf M. J. van t Erve, Chenhui Yan, Lian Li, Berry T. Jonker</b>

<i>2018-10-01</i>
> Current generated spin polarization in topological insulator (TI) surface states due to spin-momentum locking has been detected recently using ferromagnet/tunnel barrier contacts, where the projection of the TI spin onto the magnetization of the ferromagnet is measured as a voltage. However, opposing signs of the spin voltage have been reported, which had been tentatively attributed to the coexistence of trivial two-dimensional electron gas states on the TI surface which may exhibit opposite current-induced polarization than that of the TI Dirac surface states. Models based on electrochemical potential have been presented to determine the sign of the spin voltage expected for the TI surface states. However, these models neglect critical experimental parameters which also affect the sign measured. Here we present a Mott two-spin current resistor model which takes into account these parameters such as spin-dependent interface resistances, and show that such inclusion can lead to a crossing of the voltage potential profiles for the spin-up and spin-down electrons within the channel, which can lead to measured spin voltages of either sign. These findings offer a resolution of the ongoing controversy regarding opposite signs of spin signal reported in the literature, and highlight the importance of including realistic experimental parameters in the model.

### [Optical orientation and alignment of excitons in direct and indirect  band gap (In,Al)As/AlAs quantum dots with type-I band alignment](http://arxiv.org/abs/1904.02508v2) (1904.02508v2)
<b>J. Rautert, T. S. Shamirzaev, S. V. Nekrasov, D. R. Yakovlev, P. Klenovský, Yu. G. Kusrayev, M. Bayer</b>

<i>2019-04-04</i>
> The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a ![](http://latex2png.com/output//latex_df1df538394876e49bbbda0407a2ef3c.png)-valley electron and the excitons in the indirect QDs contributed by an ![](http://latex2png.com/output//latex_4296370263158f14b42066659babdfc1.png)-valley electron. We show that in the direct QDs the exciton spin dynamics is controlled by the anisotropic exchange splitting, while in the indirect QDs it is determined by the hyperfine interaction with nuclear field fluctuations. The anisotropic exchange splitting is determined for the direct QD excitons and compared with model calculations.

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