R-3 Repository :: Browsing by Author "Li, Xinwei"

Browsing by Author "Li, Xinwei"

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Band structure dependent electronic localization in macroscopic films of single-chirality single-wall carbon nanotubes
(Elsevier, 2021) Gao, Weilu; Adinehloo, Davoud; Li, Xinwei; Mojibpour, Ali; Yomogida, Yohei; Hirano, Atsushi; Tanaka, Takeshi; Kataura, Hiromichi; Zheng, Ming; Perebeinos, Vasili; Kono, Junichiro
Significant understanding has been achieved over the last few decades regarding chirality-dependent properties of single-wall carbon nanotubes (SWCNTs), primarily through single-tube studies. However, macroscopic manifestations of chirality dependence have been limited, especially in electronic transport, despite the fact that such distinct behaviors are needed for many applications of SWCNT-based devices. In addition, developing reliable transport theory is challenging since a description of localization phenomena in an assembly of nanoobjects requires precise knowledge of disorder on multiple spatial scales, particularly if the ensemble is heterogeneous. Here, we report an observation of pronounced chirality-dependent electronic localization in temperature and magnetic field dependent conductivity measurements on macroscopic films of single-chirality SWCNTs. The samples included large-gap semiconducting (6,5) and (10,3) films, narrow-gap semiconducting (7,4) and (8,5) films, and armchair metallic (6,6) films. Experimental data and theoretical calculations revealed Mott variable-range-hopping dominated transport in all samples, while localization lengths fall into three distinct categories depending on their band gaps. Armchair films have the largest localization length. Our detailed analyses on electronic transport properties of single-chirality SWCNT films provide significant new insight into electronic transport in ensembles of nanoobjects, offering foundations for designing and deploying macroscopic SWCNT solid-state devices.
Continuous transition between weak and ultrastrong coupling through exceptional points in carbon nanotube microcavity exciton–polaritons
(Springer Nature, 2018) Gao, Weilu; Li, Xinwei; Bamba, Motoaki; Kono, Junichiro
Non-perturbative coupling of photons and excitons produces hybrid particles, exciton–polaritons, which have exhibited a variety of many-body phenomena in various microcavity systems. However, the vacuum Rabi splitting (VRS), which defines the strength of photon–exciton coupling, is usually a single constant for a given system. Here, we have developed a unique architecture in which excitons in an aligned single-chirality carbon nanotube film interact with cavity photons in polarization-dependent manners. The system reveals ultrastrong coupling (VRS up to 329 meV or a coupling-strength-to-transition-energy ratio of 13.3%) for polarization parallel to the nanotube axis, whereas VRS is absent for perpendicular polarization. Between these two extremes, VRS is continuously tunable through polarization rotation with exceptional points separating crossing and anticrossing. The points between exceptional points form equienergy arcs onto which the upper and lower polaritons coalesce. The demonstrated on-demand ultrastrong coupling provides ways to explore topological properties of polaritons and quantum technology applications.
Evidence for a topological excitonic insulator in InAs/GaSb bilayers
(Springer Nature, 2017) Du, Lingjie; Li, Xinwei; Lou, Wenkai; Sullivan, Gerard; Chang, Kai; Kono, Junichiro; Du, Rui-Rui
Electron-hole pairing can occur in a dilute semimetal, transforming the system into an excitonic insulator state in which a gap spontaneously appears at the Fermi surface, analogous to a Bardeen-Cooper-Schrieffer (BCS) superconductor. Here, we report optical spectroscopic and electronic transport evidence for the formation of an excitonic insulator gap in an inverted InAs/GaSb quantum-well system at low temperatures and low electron-hole densities. Terahertz transmission spectra exhibit two absorption lines that are quantitatively consistent with predictions from the pair-breaking excitation dispersion calculated based on the BCS gap equation. Low-temperature electronic transport measurements reveal a gap of ~2 meV (or ~25 K) with a critical temperature of ~10 K in the bulk, together with quantized edge conductance, suggesting the occurrence of a topological excitonic insulator phase.
Magnonic superradiant phase transition
(Springer Nature, 2022) Bamba, Motoaki; Li, Xinwei; Marquez Peraca, Nicolas; Kono, Junichiro
In the superradiant phase transition (SRPT), coherent light and matter fields are expected to appear spontaneously in a coupled light–matter system in thermal equilibrium. However, such an equilibrium SRPT is forbidden in the case of charge-based light–matter coupling, known as no-go theorems. Here, we show that the low-temperature phase transition of ErFeO3 at a critical temperature of approximately 4 K is an equilibrium SRPT achieved through coupling between Fe3+ magnons and Er3+ spins. By verifying the efficacy of our spin model using realistic parameters evaluated via terahertz magnetospectroscopy and magnetization experiments, we demonstrate that the cooperative, ultrastrong magnon–spin coupling causes the phase transition. In contrast to prior studies on laser-driven non-equilibrium SRPTs in atomic systems, the magnonic SRPT in ErFeO3 occurs in thermal equilibrium in accordance with the originally envisioned SRPT, thereby yielding a unique ground state of a hybrid system in the ultrastrong coupling regime.
Probing Cooperative Phenomena in Solids by Terahertz Magnetospectroscopy
(2019-07-01) Li, Xinwei; Kono, Junichiro
Cooperative phenomena emerge in condensed matter when interactions between the constituent particles significantly impact, or even govern, the quantum dynamics of the system. The system then exhibits phases and properties that are undescribable by single-particle theories. For understanding the physics behind cooperative phenomena, it is important to disentangle the interplay between different degrees of freedom, including spin, charge, lattice, and orbit. In this dissertation work, we demonstrated that terahertz (THz) magnetospectrosocpy is a powerful tool for probing and elucidating cooperative phenomena in solids by studying four different types of quantum materials. First, we observed a narrow-band THz gain peak in a dense two-dimensional magnetoexciton gas in a photoexcited semiconductor quantum well. Second, by using THz polarimetry, we probed the collective magnetooptical response of surface carriers in a topological insulator in pulsed high magnetic fields. Third, we detected singular charge fluctuations in a quantum critical heavy-fermion metal in the form of ``$\omega/T$-scaling" in THz conductivity. Finally, we discovered that the exchange coupling between two spin systems in a magnetic insulator follows the scaling behavior expected for Dicke cooperativity, a well-kown many-body process in quantum optics. The experiments and analyses in these studies can be extended to a variety of other quantum materials for advancing the understanding of many-body physics.
Quantum simulation of an extended Dicke model with a magnetic solid
(Springer Nature, 2024) Marquez Peraca, Nicolas; Li, Xinwei; Moya, Jaime M.; Hayashida, Kenji; Kim, Dasom; Ma, Xiaoxuan; Neubauer, Kelly J.; Fallas Padilla, Diego; Huang, Chien-Lung; Dai, Pengcheng; Nevidomskyy, Andriy H.; Pu, Han; Morosan, Emilia; Cao, Shixun; Bamba, Motoaki; Kono, Junichiro
The Dicke model describes the cooperative interaction of an ensemble of two-level atoms with a single-mode photonic field and exhibits a quantum phase transition as a function of light–matter coupling strength. Extending this model by incorporating short-range atom–atom interactions makes the problem intractable but is expected to produce new physical phenomena and phases. Here, we simulate such an extended Dicke model using a crystal of ErFeO3, where the role of atoms (photons) is played by Er3+ spins (Fe3+ magnons). Through terahertz spectroscopy and magnetocaloric effect measurements as a function of temperature and magnetic field, we demonstrated the existence of a novel atomically ordered phase in addition to the superradiant and normal phases that are expected from the standard Dicke model. Further, we elucidated the nature of the phase boundaries in the temperature–magnetic-field phase diagram, identifying both first-order and second-order phase transitions. These results lay the foundation for studying multiatomic quantum optics models using well-characterized many-body solid-state systems.
Terahertz Faraday and Kerr rotation spectroscopy of Bi1−xSbx films in high magnetic fields up to 30 tesla
(American Physical Society, 2019) Li, Xinwei; Yoshioka, Katsumasa; Xie, Ming; Noe, G. Timothy; Lee, Woojoo; Marquez Peraca, Nicolas; Gao, Weilu; Hagiwara, Toshio; Handegård, Ørjan S.; Nien, Li-Wei; Nagao, Tadaaki; Kitajima, Masahiro; Nojiri, Hiroyuki; Shih, Chih-Kang; MacDonald, Allan H.; Katayama, Ikufumi; Takeda, Jun; Fiete, Gregory A.; Kono, Junichiro
We report results of terahertz Faraday and Kerr rotation spectroscopy measurements on thin films of Bi1−xSbx, an alloy system that exhibits a semimetal-to-topological-insulator transition as the Sb composition x increases. By using a single-shot time-domain terahertz spectroscopy setup combined with a table-top pulsed minicoil magnet, we conducted measurements in magnetic fields up to 30 T, observing distinctly different behaviors between semimetallic (x<0.07) and topological insulator (x>0.07) samples. Faraday and Kerr rotation spectra for the semimetallic films showed a pronounced dip that blueshifted with the magnetic field, whereas spectra for the topological insulator films were positive and featureless, increasing in amplitude with increasing magnetic field and eventually saturating at high fields (>20 T). Ellipticity spectra for the semimetallic films showed resonances, whereas the topological insulator films showed no detectable ellipticity. To explain these observations, we developed a theoretical model based on realistic band parameters and the Kubo formula for calculating the optical conductivity of Landau-quantized charge carriers. Our calculations quantitatively reproduced all experimental features, establishing that the Faraday and Kerr signals in the semimetallic films predominantly arise from bulk hole cyclotron resonances while the signals in the topological insulator films represent combined effects of surface carriers originating from multiple electron and hole pockets. These results demonstrate that the use of high magnetic fields in terahertz magnetopolarimetry, combined with detailed electronic structure and conductivity calculations, allows us to unambiguously identify and quantitatively determine unique contributions from different species of carriers of topological and nontopological nature in Bi1−xSbx.
Ultrastrong magnon–magnon coupling dominated by antiresonant interactions
(Springer Nature, 2021) Makihara, Takuma; Hayashida, Kenji; Noe Ii, G. Timothy; Li, Xinwei; Marquez Peraca, Nicolas; Ma, Xiaoxuan; Jin, Zuanming; Ren, Wei; Ma, Guohong; Katayama, Ikufumi; Takeda, Jun; Nojiri, Hiroyuki; Turchinovich, Dmitry; Cao, Shixun; Bamba, Motoaki; Kono, Junichiro
Exotic quantum vacuum phenomena are predicted in cavity quantum electrodynamics systems with ultrastrong light-matter interactions. Their ground states are predicted to be vacuum squeezed states with suppressed quantum fluctuations owing to antiresonant terms in the Hamiltonian. However, such predictions have not been realized because antiresonant interactions are typically negligible compared to resonant interactions in light-matter systems. Here we report an unusual, ultrastrongly coupled matter-matter system of magnons that is analytically described by a unique Hamiltonian in which the relative importance of resonant and antiresonant interactions can be easily tuned and the latter can be made vastly dominant. We found a regime where vacuum Bloch-Siegert shifts, the hallmark of antiresonant interactions, greatly exceed analogous frequency shifts from resonant interactions. Further, we theoretically explored the system’s ground state and calculated up to 5.9 dB of quantum fluctuation suppression. These observations demonstrate that magnonic systems provide an ideal platform for exploring exotic quantum vacuum phenomena predicted in ultrastrongly coupled light-matter systems.
Vacuum Bloch-Siegert Shift in Landau Polaritons with Ultrahigh Cooperativity
(2018-02-02) Li, Xinwei; Kono, Junichiro
A two-level system resonantly interacting with an ac magnetic or electric field constitutes the physical basis of diverse phenomena and technologies, including nuclear magnetic resonance, stimulated emission, amplification, Rabi oscillations, laser cooling, and quantum information processing. However, despite the seeming simplicity of the problem, Schrodinger's equation for this system can be solved exactly only under the rotating wave approximation, which neglects the counter-rotating field component. When the ac field is sufficiently strong, this approximation fails, leading to a resonance-frequency shift known as the Bloch-Siegert (BS) shift, which is typically minuscule and difficult to analyze. Here, we report the vacuum BS shift, which is induced by the ultrastrong coupling of matter with the counter-rotating component of the vacuum fluctuation field in a cavity. Specifically, an ultrahigh-mobility 2D electron gas inside a high-Q terahertz cavity in a quantizing magnetic field revealed Landau polaritons with a record high value (3513) of the ratio of the vacuum Rabi splitting to the polariton linewidth. Unlike the usual BS shift, we observed an unambiguously large vacuum BS shift up to 40 GHz, which can be exactly analyzed as a consequence of the ultrastrong coupling of counter-rotating circularly polarized radiation and Landau-quantized electrons. This shift, clearly distinguishable from the photon-field self-interaction effect, represents a unique manifestation of a strong-field phenomenon without a strong field.
Vacuum Bloch–Siegert shift in Landau polaritons with ultra-high cooperativity
(Springer Nature, 2018) Li, Xinwei; Bamba, Motoaki; Zhang, Qi; Fallahi, Saeed; Gardner, Geoff C.; Gao, Weilu; Lou, Minhan; Yoshioka, Katsumasa; Manfra, Michael J.; Kono, Junichiro
A two-level system resonantly interacting with an a.c. magnetic or electric field constitutes the physical basis of diverse phenomena and technologies. However, Schrödinger’s equation for this seemingly simple system can be solved exactly only under the rotating-wave approximation, which neglects the counter-rotating field component. When the a.c. field is sufficiently strong, this approximation fails, leading to a resonance-frequency shift known as the Bloch–Siegert shift. Here, we report the vacuum Bloch–Siegert shift, which is induced by the ultra-strong coupling of matter with the counter-rotating component of the vacuum fluctuation field in a cavity. Specifically, an ultra-high-mobility two-dimensional electron gas inside a high-Qterahertz cavity in a quantizing magnetic field revealed ultra-narrow Landau polaritons, which exhibited a vacuum Bloch–Siegert shift up to 40 GHz. This shift, clearly distinguishable from the photon-field self-interaction effect, represents a unique manifestation of a strong-field phenomenon without a strong field.

Browsing by Author "Li, Xinwei"

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