_ABOUT

Nicodemos Varnava

I’m a Condensed Matter Theorist currently working towards my PhD at Rutgers University under Prof. David Vanderbilt.

In 2016, I graduated summa cum laude from the National Technical University of Athens with a 5-year joint degree, specializing in theoretical physics and advance material science.

I’m currently interested in the field of topological materials. My research revolves around classifying and predicting new topological materials as well as exploring novel phenomena and applications of existing topological materials.

  Open CV

_RESEARCH

Topological Phases of Matter

These are new phases of matter that defy the conventional spontaneous symmetry broking mechanism. Instead certain properties are insensitive to smooth changes in material parameters and cannot change unless the system passes through a quantum phase transition. In that sense topological materials are unique since their responses can be quantized with extreme precision.

Ab-Initio Calculations

Ab-initio (i.e. "from the beginning") calculations are simulations that are based on the laws of quantum mechanics and only use the fundamental constants of physics as input to provide detailed insight into the origin of mechanical, electronic, optical and magnetic properties of materials and molecules. In recent years they brought theory and experiment hand to hand and made condensed matter physics the most rapidly evolving research field in physics.

_PUBLICATIONS

QPJ - Controlling a quantum point junction

Controlling a quantum point junction on the surface of an antiferromagnetic topological insulator

N. Varnava, J.H. Wilson, J.H. Pixley, and D. Vanderbilt, ``Controlling a quantum point junction on the surface of an antiferromagnetic topological insulator'', arXiv:2008.03316

The surface and edges of topological materials can host physics, such as unidirectional charge or spin transport, that is unavailable in isolated one- and two-dimensional systems. However, to fully control the mixing and interference of edge-state wave functions, one needs robust and tunable junctions. We propose to achieve this control using an antiferromagnetic topological insulator that supports two distinct types of gapless unidirectional channels on its surface, one from antiferromagnetic domain walls and the other from single-height steps. The distinct geometric nature of these edge modes allows them to intersect robustly to form quantum point junctions, and their presence at the surface makes them subject to control by magnetic and electrostatic tips like those used in scanning probe microscopes. Prospects for realizing such junctions are encouraged by recent material candidate proposals, potentially leading to exciting applications in quantum computing and sensing.

Axion coupling in the hybrid Wannier representation

Axion coupling in the hybrid Wannier representation

N. Varnava, I. Souza, and D. Vanderbilt,``Axion coupling in the hybrid Wannier representation'', PhysRevB.101.155130

Many magnetic point-group symmetries induce a topological classification on crystalline insulators, dividing them into those that have a nonzero quantized Chern-Simons magnetoelectric coupling (“axion-odd” or “topological”), and those that do not (“axion-even” or “trivial”). For time-reversal or inversion symmetries, the resulting topological state is usually denoted as a “strong topological insulator” or an “axion insulator” respectively, but many other symmetries can also protect this “axion Z2” index. Topological states are often insightfully characterized by inspecting the hybrid Wannier (or equivalently, the non-Abelian Wilson-loop) band structure. Here, we systematically classify the axion-quantizing symmetries, and explore the implications of such symmetries on the Wannier band structure. Conversely, we clarify the conditions under which the axion Z2 index can be deduced from the Wannier band structure.

Surfaces Of Axion Insulators

Surfaces Of Axion Insulators

N. Varnava and D. Vanderbilt, ``Surfaces of Axion Insulators'', PhysRevB.98.245117

Axion insulators are magnetic topological insulators in which the non-trivial ℤ2 index is protected by inversion symmetry instead of time-reversal symmetry. The naturally gapped surfaces of axion insulators give rise to a half-quantized surface anomalous Hall conductivity (AHC), but its sign cannot be determined from topological arguments. In this paper, we consider topological phenomena at the surface of an axion insulator. We also implement a recently proposed approach for calculating the surface AHC directly, which allows us to explore how the interplay between surface termination and magnetic ordering determines the sign of the half-quantized surface AHC. We show that it is possible to construct a topological state with no protected metallic states on boundaries of any dimension (surfaces, hinges, or corners), although chiral hinge modes do occur for many surface configurations. In the ferromagnetic configuration, rotation of the magnetization by an external field offers a promising means of control of chiral hinge modes, which can also appear on surface steps or where bulk domain walls emerge at the surface.

_CONTACT

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