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Kenan Gundogdu

Publications and source records attributed to Kenan Gundogdu.

9 recordsLinked to original sources
arXiv2026

Emergence of Macroscopic Quantum Order via Translational Zero Modes

Macroscopic quantum coherence in solids, such as in superfluids, superconductors, and condensates, is generally limited to low temperatures because order forms within a fixed excitation spectrum whose competing states become thermally populated as temperature rises. Here, we show that strong coupling between electronic excitations and a deformable lattice enables a different route. Above a critical density, this coupling nucleates self generated confining potentials that trap the very excitations generating them. Unlike rigid external traps, these potentials can translate through the host lattice without changing their internal structure, defining a translational zero mode. Coupling to this zero mode provides a shared dynamical coordinate that lowers and isolates a single collective many body configuration, opening a density dependent gap that suppresses thermal occupation of competing states and supports off diagonal long range order at elevated temperatures. As a concrete realization, we identify high-temperature superfluorescence in lead halide perovskites as the radiative instability of this zero mode dressed ordered excitonic state. More broadly, this establishes a general route to macroscopic quantum order: not cooling within a fixed spectrum, nor pairing instabilities, but a self generated mobile confining structure whose translational zero mode reconstructs the many-body spectrum to protect coherence.

cond-mat.mtrl-sci↗
arXiv2025

THz Emission from Spintronic Microstructure

Recent advancements in spintronics have opened a new avenue in terahertz (THz) radiation sources that may outperform the traditional contact-based metallic counterparts. Inspired by the generation of broadband spintronic THz signals at the interface of a ferromagnet and ultrawide bandgap semiconductors, here we investigated the generation of THz radiation from micro-structured heterostructures of a metallic ferromagnet (Ni80Fe20) and an ultrawide bandgap semiconductor (AlGaN/GaN) that contains a layer of 2D electron gas. By precisely tailoring the dimension of the subwavelength pillars of a THz device, the micro-structured spintronic THz emitter can achieve up to more than three times higher emission intensity compared to that of the un-patterned counterpart. Our study advances the development of the next generation of spintronic THz sources that allow a tailored emission frequency and intensity control and, further, are compatible with existing integrated wide-bandgap semiconductor circuits.

cond-mat.mes-hall↗
arXiv2022

Quantum Analog of Vibration Isolation: From Room Temperature Superfluorescence to High Temperature Superconductivity

The development and the use of quantum technologies are hindered by a fundamental challenge: Quantum materials exhibit macroscopic quantum properties at extremely low temperatures due to the loss of quantum coherence at elevated temperatures. Here, based on our recent discovery of room temperature superfluorescence in perovskites, we present the Quantum Analog of Vibration Isolation, 'QAVI', model and explain how it protects the quantum phase against dephasing at high temperatures. We then postulate the requirements for observation of macroscopic quantum phenomena at practical temperatures and propose a unified model for all macroscopic quantum phase transitions. We further present the general features of the temperature and density phase diagram of macroscopic quantum phase transitions that are mediated by the QAVI process and identify the similarities observed in the phase diagram of high Tc superconductors. Understanding this fundamental quantum coherence protection mechanism is imperative to accelerate the discovery of high temperature macroscopic quantum phenomena, and offers significant potential for developing quantum technologies functioning under practical conditions.

cond-mat.supr-con↗
arXiv2019

Room-Temperature Electron-Hole Liquid in Monolayer MoS2

Excitons in semiconductors are usually non interacting and behave like an ideal gas, but may condense to a strongly correlated liquid like state, i.e. electron hole liquid (EHL), at high density and appropriate temperature. EHL is a macroscopic quantum state with exotic properties and represents the ultimate attainable charge excitation density in steady states. It bears great promise for a variety of fields such as ultrahigh power photonics and quantum science and technology. However, the condensation of gas like excitons to EHL has often been restricted to cryogenic temperatures, which significantly limits the prospect of EHL for use in practical applications. Herein we demonstrate the formation of EHL at room temperature in monolayer MoS2 by taking advantage of the monolayer's extraordinarily strong exciton binding energy. This work demonstrates the potential for the liquid like state of charge excitations to be a useful platform for the studies of macroscopic quantum phenomena and the development of optoelectronic devices.

cond-mat.mtrl-sci↗
arXiv2017

Enhancing Multifunctionalities of Transition Metal Dichalcogenide Monolayers via Intercalation of Molecules and Ions

Transition metal dichalcogenide (TMDC) monolayers present a remarkable multifunctional material with potential to enable the development of a wide range of novel devices. However, the functionalities observed often fall short of the expectation, which hinders the device development. Here we demonstrate that the optical, catalytic, and thermal functionalities of TMDC monolayers can all be substantially enhanced by up to orders of magnitude with the intercalation of water molecules or small cations (H+ and Li+) between the monolayers and underlying substrates. In contrast, the same molecules or cations adsorbed on top of the monolayers show negligible effects. We also discover two major roles of the intercalated species in the enhancement: doping the monolayers and modifying the interaction of the monolayers with the substrate. The result points out a versatile and convenient strategy of using the intercalation of molecules or ions to enhance the functionalities of TMDC monolayers.

cond-mat.mtrl-sci↗
arXiv2016

Fundamental Limits of Exciton-Exciton Annihilation for Light Emission in Transition Metal Dichalcogenide Monolayers

We quantitatively illustrate the fundamental limit that exciton-exciton annihilation (EEA) may impose to the light emission of monolayer transition metal dichalcogenide (TMDC) materials. The EEA in TMDC monolayers shows dependence on the interaction with substrates as its rate increases from 0.1 cm2/s (0.05 cm2/s) to 0.3 cm2/s (0.1 cm2/s) with the substrates removed for WS2 (MoS2) monolayers. It turns to be the major pathway of exciton decay and dominates the luminescence efficiency when the exciton density is beyond 1010 cm-2 in suspended monolayers or 1011 cm-2 in supported monolayers. This sets an upper limit on the density of injected charges in light emission devices for the realization of optimal luminescence efficiency. The strong EEA rate also dictates the pumping threshold for population inversion in the monolayers to be 12-18 MW/cm2 (optically) or 2.5-4x105 A/cm2 (electrically).

physics.optics↗
arXiv2014

Temperature Dependent Valley Relaxation Dynamics in Single Layer WS2 Measured Using Ultrafast Spectroscopy

We measured the lifetime of optically created valley polarization in single layer WS2 using transient absorption spectroscopy. The electron valley relaxation is very short (< 1ps). However the hole valley lifetime is at least two orders of magnitude longer and exhibits a temperature dependence that cannot be explained by single carrier spin/valley relaxation mechanisms. Our theoretical analysis suggests that a collective contribution of two potential processes may explain the valley relaxation in single layer WS2. One process involves direct scattering of excitons from K to K' valleys with a spin flip-flop interaction. The other mechanism involves scattering through spin degenerate Gamma valley. This second process is thermally activated with an Arrhenius behavior due to the energy barrier between Gamma and K valleys.

cond-mat.mtrl-sci↗
arXiv2010

Bond-specific reaction kinetics during the oxidation of (111) Si: Effect of n-type doping

It is known that a higher concentration of free carriers leads to a higher oxide growth rate in the thermal oxidation of silicon. However, the role of electrons and holes in oxidation chemistry is not clear. Here, we report real-time second-harmonic-generation data on the oxidation of H-terminated (111)Si that reveal that high concentrations of electrons increase the chemical reactivity of the outer-layer Si-Si back bonds relative to the Si-H up bonds. However, the thicknesses of the natural oxides of all samples stabilize near 1 nm at room temperature, regardless of the chemical kinetics of the different bonds.

cond-mat.mtrl-sci↗