arXiv subjects

Vladimir Y. Umansky

Publications and source records attributed to Vladimir Y. Umansky.

3 recordsLinked to original sources

arXiv2026

Delayed Formation of Landau Polaritons in Phase-Resolved THz Spectroscopy

Strong light-matter coupling gives rise to polaritons through coherent and periodic energy exchange between electromagnetic cavity fields and material excitations. While this interaction is typically inferred from spectral mode splitting, its dynamics remain largely unexplored. Here, using phase-resolved terahertz time-domain spectroscopy, we observe Rabi oscillations of Landau polaritons formed by coupling the cyclotron resonance in a GaAs/Al$_{0.36}$Ga$_{0.64}$As two-dimensional electron gas with Fabry-Perot cavity modes. By employing cross-polarized spectroscopy and magnetic-field differential, we resolve the temporal beating of the cyclotron resonance oscillations. Remarkably, we find that the Rabi oscillations do not start immediately after excitation of the cyclotron resonance, but after a delay corresponding to one cavity round-trip time. This demonstrates that the strong-coupling regime sets up only after the formation of the cavity mode field. Our results provide direct insight into the dynamics of hybrid light-matter states in the THz regime.

cond-mat.mes-hall↗

arXiv2025

Nature of current flow in the regime of the quantum Hall effect

The integer quantum Hall effect (QHE) belongs to the most fundamental phenomena of solid state physics and has an important application as resistance standard. It serves as a basis to understand the fractional, anomalous or spin QHEs, candidates for applications in quantum technology due to their topological properties. For optimizing all these applications it is essential to understand where the current flows inside the Hall bar, a question disputed for decades. We perform multiterminal current measurements on a Hall bar and compare the results with limiting models. We confirm, based on these experiments, that the current flow is chiral for the plateaus of quantized Hall resistance. Everywhere else, including the ranges between plateaus, our results comply with the Drude model, which predicts homogeneous current flow across a homogeneous Hall-bar.

cond-mat.mes-hall↗

arXiv2025

Transition from edge- to bulk-currents in the quantum Hall regime

The integer quantum Hall effect can be observed in a two-dimensional conductor penetrated by a perpendicular magnetic field and with edges connecting the current carrying contacts. Its signature is a state of quantized Hall and simultaneously vanishing longitudinal resistances. A widely accepted model is the Landauer-Büttiker picture, which assumes an incompressible, i.e., electrically insulating bulk state surrounded by current carrying one-dimensional edge channels. This single-particle model is challenged by the screening theory. It derives, that electron-electron interaction leads to a fragmentation of the Hall bar into compressible and incompressible strips, where the current flows inside the incompressible strips. Because the latter gradually shift from the sample edges into the bulk as the magnetic field is increased, it suggests a transition from edge- to bulk-current. We present a direct experimental proof of this transition. Our results support the screening theory.

cond-mat.mes-hall↗