Abstract
Magnetic layered van der Waals crystals are an emerging class of materials giving access to new physical phenomena, as illustrated by the recent observation of 2D ferromagnetism in Cr
2
Ge
2
...Te
6
and CrI
3
. Of particular interest in semiconductors is the interplay between magnetism and transport, which has remained unexplored. Here we report magneto-transport measurements on exfoliated CrI
3
crystals. We find that tunneling conduction in the direction perpendicular to the crystalline planes exhibits a magnetoresistance as large as 10,000%. The evolution of the magnetoresistance with magnetic field and temperature reveals that the phenomenon originates from multiple transitions to different magnetic states, whose possible microscopic nature is discussed on the basis of all existing experimental observations. This observed dependence of the conductance of a tunnel barrier on its magnetic state is a phenomenon that demonstrates the presence of a strong coupling between transport and magnetism in magnetic van der Waals semiconductors.
Strongly correlated photons on a chip Reinhard, Andreas; Volz, Thomas; Winger, Martin ...
Nature photonics,
02/2012, Volume:
6, Issue:
2
Journal Article
Peer reviewed
Optical nonlinearities at the single-photon level are key ingredients for future photonic quantum technologies. Prime candidates for the realization of the strong photon-photon interactions necessary ...for implementing quantum information processing tasks, as well as for studying strongly correlated photons in an integrated photonic device setting, are quantum dots embedded in photonic-crystal nanocavities. Here, we report strong quantum correlations between photons on picosecond timescales. We observe (i) photon antibunching upon resonant excitation of the lowest-energy polariton state, proving that the first cavity photon blocks the subsequent injection events, and (ii) photon bunching when the laser field is in two-photon resonance with the polariton eigenstates of the second Jaynes-Cummings manifold, demonstrating that two photons at this colour are more likely to be injected into the cavity jointly than they would otherwise. Together, these results demonstrate unprecedented strong single-photon nonlinearities, paving the way for the realization of a quantum optical Josephson interferometer or a single-photon transistor.
An as yet outstanding goal in quantum optics is the realization of fast optical nonlinearities at the single-photon level. This would allow for the implementation of optical devices with new ...functionalities such as single-photon switches/transistors or controlled-phase gates. Although nonlinear optics effects at the single-emitter level have been demonstrated in a number of systems, none of these experiments showed single-photon switching on ultrafast timescales. Here, we perform pulsed two-colour spectroscopy and demonstrate that, in a strongly coupled quantum dot-cavity system, the presence of a single photon on one of the fundamental polariton transitions can turn on light scattering on a transition from the first to the second Jaynes-Cummings manifold. The overall switching time of this single-photon all-optical switch is ∼50 ps. In addition, we use the single-photon nonlinearity to implement a pulse correlator. Our quantum dot-cavity system could form the building block of future high-bandwidth photonic networks operating in the quantum regime.
When the Coulomb repulsion between electrons dominates over their kinetic energy, electrons in two-dimensional systems are predicted to spontaneously break continuous-translation symmetry and form a ...quantum crystal1. Efforts to observe2-12 this elusive state of matter, termed a Wigner crystal, in two-dimensional extended systems have primarily focused on conductivity measurements on electrons confined to a single Landau level at high magnetic fields. Here we use optical spectroscopy to demonstrate that electrons in a monolayer semiconductor with density lower than 3 x 1011 per centimetre squared form a Wigner crystal. The combination of a high electron effective mass and reduced dielectric screening enables us to observe electronic charge order even in the absence of a moiré potential or an external magnetic field. The interactions between a resonantly injected exciton and electrons arranged in a periodic lattice modify the exciton bandstructure so that an umklapp resonance arises in the optical reflection spectrum, heralding the presence of charge order13. Our findings demonstrate that charge-tunable transition metal dichalcogenide monolayers14 enable the investigation of previously uncharted territory for many-body physics where interaction energy dominates over kinetic energy.
The elementary optical excitations in two-dimensional semiconductors hosting itinerant electrons are attractive and repulsive polarons—excitons that are dynamically screened by electrons. Exciton ...polarons have hitherto been studied in translationally invariant degenerate Fermi systems. Here, we show that periodic distribution of electrons breaks the excitonic translational invariance and leads to a direct optical signature in the exciton-polaron spectrum. Specifically, we demonstrate that new optical resonances appear due to spatially modulated interactions between excitons and electrons in an incompressible Mott-like correlated state. Our observations demonstrate that resonant optical spectroscopy provides an invaluable tool for studying strongly correlated states, such as Wigner crystals and density waves, where exciton-electron interactions are modified by the emergence of charge order.
In a coupled quantum-dot-nanocavity system, the photoluminescence from an off-resonance cavity mode exhibits strong quantum correlations with the quantum-dot transitions, even though its ...autocorrelation function is classical. Using new pump-power dependent photon-correlation measurements, we demonstrate that this seemingly contradictory observation that has so far defied an explanation stems from cascaded cavity photon emission in transitions between excited multiexciton states. The mesoscopic nature of quantum-dot confinement ensures the presence of a quasicontinuum of excitonic transitions, part of which overlaps with the cavity resonance.
We study kinetic magnetism for the Fermi-Hubbard model in triangular lattices. We focus on the regime of strong interactions, U≫t, and filling factors around one electron per site. For temperatures ...well above the hopping strength t, the Curie-Weiss form of the magnetic susceptibility suggests two complementary forms of kinetic magnetism. In the case of hole doping, antiferromagnetic polarons originate from kinetic frustration of individual holes, whereas for electron doping, Nagaoka-type ferromagnetic correlations are induced by propagating doublons. These results provide a possible theoretical explanation of recent experimental results in moiré transition metaldichalcogenide materials and cold atom systems. To understand many-body states arising from antiferromagentic polarons at low temperatures, we study hole-doped systems in finite magnetic fields. At low dopings and intermediate magnetic fields, we find a magnetic polaron phase, separated from the fully polarized state by a metamagnetic transition. With decreasing magnetic field, the system shows a tendency to phase separate with hole-rich regions forming antiferromagnetic spin bags. We demonstrate that direct observations of magnetic polarons in triangular lattices can be achieved in experiments with ultracold atoms, which allow measurements of three point hole-spin-spin correlations.
We demonstrate that electron-phonon interaction in quantum dots embedded in one-dimensional systems leads to pronounced, non-Markovian decoherence of optical transitions. The experiments that we ...present focus on the line shape of photoluminescence from low-temperature axially localized carbon nanotube excitons. The independent boson model that we use to model the phonon interactions reproduces with very high accuracy the broad and asymmetric emission lines and the weak red-detuned radial breathing mode replicas observed in the experiments. The intrinsic phonon-induced pure dephasing of the zero-phonon line is 2 orders of magnitude larger than the lifetime broadening and is a hallmark of the reduced dimensionality of the phonon bath. The non-Markovian nature of this decoherence mechanism may have adverse consequences for applications of one-dimensional systems in quantum information processing.
Cavity-polaritons in semiconductor microstructures have emerged as a promising system for exploring non-equilibrium dynamics of many-body systems
. Key advances in this field, including the ...observation of polariton condensation
, superfluidity
, realization of topological photonic bands
, and dissipative phase transitions
, generically allow for a description based on a mean-field Gross-Pitaevskii formalism. Observation of polariton intensity squeezing
and decoherence of a polarization entangled photon pair by a polariton condensate
, on the other hand, demonstrate quantum effects that show up at high polariton occupancy. Going beyond and into the regime of strongly correlated polaritons requires the observation of a photon blockade effect
where interactions are strong enough to suppress double occupancy of a photonic lattice site. Here, we report evidence of quantum correlations between polaritons spatially confined in a fibre cavity. Photon correlation measurements show that careful tuning of the coupled system can lead to a modest reduction of simultaneous two-polariton generation probability by 5%. Concurrently, our experiments allow us to measure the polariton interaction strength, thereby resolving the controversy stemming from recent experimental reports
. Our findings constitute an essential step towards the realization of strongly interacting photonic systems.
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