arXiv subjects

Robert Markiewicz

Publications and source records attributed to Robert Markiewicz.

5 recordsLinked to original sources

arXiv2025

Giant Kohn anomaly and chiral phonons in the charge density wave phase of 1H-NbSe$_2$

Despite extensive investigations, many aspects of charge density waves (CDWs) remain elusive, especially the relative roles of electron-phonon coupling and Fermi surface nesting as the underlying driving mechanisms responsible for the emergence of the CDW vector $Q_{CDW}$. It is puzzling that even though electrons interact strongly with optical phonons in many correlated systems, the actual mode softening is of an acoustic mode. Here we consider monolayer 1H-NbSe$_2$ as an exemplar system, and through an accurate computation of the phonon self-energy, including its off-diagonal components, we provide compelling evidence that the relevant mode is a longitudinal optical phonon that softens by anti-crossing several intervening phonon bands, i.e. a Kohn ladder which has been only observed previously in high temperature superconductors. We also show that $Q_{CDW}$ is fixed by the convolution of the susceptibility and electron-phonon coupling, and that the softened phonons are circularly polarized.

cond-mat.str-el↗

arXiv2022

Relativistic horizon of interacting Weyl fermions in condensed matter systems

The intersections of topology, geometry and strong correlations offer many opportunities for exotic quantum phases to emerge in condensed matter systems. Weyl fermions, in particular, provide an ideal platform for exploring the dynamical instabilities of single-particle physics under interactions. Despite its fundamental role in relativistic field theory, the concept of causality and the associated spacetime light cone and event horizon has not been considered in connection with interacting Weyl fermionic excitations in quantum matter. Here, by using charge-density wave (CDW) as an example, we unveil the behavior of interacting Weyl fermions and show that a Weyl fermion in a system can open a band gap by interacting only with other Weyl fermions that lie within its energy-momentum dispersion cone. In this sense, causal connections or interactions are only possible within overlapping dispersion cones and each dispersion cone thus constitutes a solid-state analogue of the more conventional `event horizon' of high-energy physics. Our study provides a universal framework for considering interacting relativistic quasiparticles in condensed matter by separating them into energy-like and momentum-like relationships in analogy with the time-like and space-like events in high-energy physics. Finally, we consider two different candidate materials for hosting the Weyl CDW phase: (TaSe$_4$)$_2$I and Mo$_3$Al$_2$C. Our study greatly enriches the phenomenology and unveils new connections between condensed matter and high-energy physics.

cond-mat.str-el↗

arXiv2021

Antiferromagnetic VdW Phase at the Interface of Sputtered Topological Insulator/Ferromagnet-Bi2Te3/Ni80Fe20 Heterostructures

Magnetic ordering in topological insulators (TI) is crucial for breaking time-reversal symmetry (TRS) and thereby opening a gap in the topological surface states (TSSs) [1-6], which is the key for realizing useful topological properties such as the quantum anomalous Hall (QAH) effect, axion insulator state and the topological magnetoelectric effect. Combining TIs with magnetic materials can be expected to yield interfaces [26-28] with unique topological and magnetic phases but such interfaces largely remain unexplored. Here, we report the discovery of a novel antiferromagnetic (AFM) Van der Waals (VdW) phase at the interface of a sputtered c-axis oriented TI/FM (Bi2Te3/Ni80Fe20) heterostructure due to the formation of a Ni-intercalated Bi2Te3 VdW interfacial layer. The TI/FM heterostructure is shown to possess a significant spontaneous exchange bias and the presence of an AFM order at the interface via measurements of the hysteresis loop as well as the observation of compensated magnetic moments at the interface using polarized neutron reflectometry (PNR). An in-depth analysis of the structural and chemical properties of the interfacial AFM phase was carried out using selected area electron diffraction (SAED), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS). These studies show evidence of solid-state reaction between the intercalated Ni atoms and Bi2Te3 layers and of the formation of topologically nontrivial magnetic VdW compounds. The N\'eel temperature of the interfacial AFM phase is 63 K, which is higher than that of typical magnetic topological insulators [53]. Our study shows how industrial CMOS-process-compatible sputtered TI/FM heterostructures can provide a novel materials platform for exploring the emergence of interfacial topological magnetic phases and high-temperature topological magnetic states.

cond-mat.mtrl-sci↗

arXiv2021

Strain-Induced Charge Density Waves with Emergent Topological States in Monolayer NbSe2

Emergence of topological states in strongly correlated systems, particularly two-dimensional (2D) transition-metal dichalcogenides, offers a platform for manipulating electronic properties in quantum materials. However, a comprehensive understanding of the intricate interplay between correlations and topology remains elusive. Here we employ first-principles modeling to reveal two distinct 2x2 charge density wave (CDW) phases in monolayer 1H-NbSe2, which become energetically favorable over the conventional 3x3 CDWs under modest biaxial tensile strain of about 1%. These strain-induced CDW phases coexist with numerous topological states characterized by Z2 topology, high mirror Chern numbers, topological nodal lines, and higher-order topological states, which we have verified rigorously by computing the topological indices and the presence of robust edge states and localized corner states. Remarkably, these topological properties emerge because of the CDW rather than a pre-existing topology in the pristine phase. These results elucidate the interplay between correlations, topology, and geometry in 2D materials and indicate that strain-induced correlation effects can be used to engineer topological states in materials with initially trivial topology. Our findings may be applied in electronics, spintronics, and other advanced quantum devices that require robust and tunable topological states.

cond-mat.mtrl-sci↗

arXiv2014

Spin correlated electronic state on the surface of a spin-orbit Mott system (Layered Iridates)

Novel phases of two dimensional electron systems resulting from new surface or interface modified electronic structures have generated significant interest in material science. We utilize photoemission spectroscopy to show that the near-surface electronic structure of a bulk insulating iridate Sr$_3$Ir$_2$O$_7$ lying near metal-Mott insulator transition exhibit weak metallicity signified by finite electronic spectral weight at the Fermi level. The surface electrons exhibit a unique spin structure resulting from an interplay of spin-orbit, Coulomb interaction and surface quantum magnetism, distinct from a topological insulator state. Our results suggest the experimental realization of a novel quasi two dimensional interacting electron surface ground state, opening the door for exotic quantum entanglement and transport phenomena in iridate-based oxide devices.

cond-mat.str-el↗