Jonathan Ludwig

LaAlO 3 stoichiometry is key to electron liquid formation at LaAlO 3 /SrTiO 3 interfaces

Emergent phenomena, including superconductivity and magnetism, found in the two-dimensional electron liquid (2-DEL) at the interface between the insulators lanthanum aluminate (LaAlO3) and strontium titanate (SrTiO3) distinguish this rich system from conventional 2D electron gases at compound semiconductor interfaces. The origin of this 2-DEL, however, is highly debated, with focus on the role of defects in the SrTiO3, while the LaAlO3 has been assumed perfect. Here we demonstrate, through experiments and first-principle calculations, that the cation stoichiometry of the nominal LaAlO3 layer is key to 2-DEL formation: only Al-rich LaAlO3 results in a 2-DEL. Although extrinsic defects, including oxygen deficiency, are known to render LaAlO3/SrTiO3 samples conducting, our results show that in the absence of such extrinsic defects an interface 2-DEL can form. Its origin is consistent with an intrinsic electronic reconstruction occurring to counteract a polarization catastrophe. This work provides insight for identifying other interfaces where emergent behaviours await discovery.

High Photoresponsivity and Short Photoresponse Times in Few-Layered WSe2 Transistors

Here, we report the photoconducting response of field-effect transistors based on three atomic layers of chemical vapor transport grown WSe2 crystals mechanically exfoliated onto SiO2. We find that trilayered WSe2 field-effect transistors, built with the simplest possible architecture, can display high hole mobilities ranging from 350 cm(2)/(V s) at room temperature (saturating at a value of ∼500 cm(2)/(V s) below 50 K) displaying a strong photocurrent response, which leads to exceptionally high photoresponsivities up to 7 A/W under white light illumination of the entire channel for power densities p < 10(2) W/m(2). Under a fixed wavelength of λ = 532 nm and a laser spot size smaller than the conducting channel area, we extract photoresponsitivities approaching 100 mA/W with concomitantly high external quantum efficiencies up to ∼40% at room temperature. These values surpass values recently reported from more complex architectures, such as graphene and transition metal dichalcogenides based heterostructures. Also, trilayered WSe2 phototransistors display photoresponse times on the order of 10 μs. Our results indicate that the addition of a few atomic layers considerably decreases the photoresponse times, probably by minimizing the interaction with the substrates, while maintaining a very high photoresponsivity.

Pronounced Photovoltaic Response from Multilayered Transition-Metal Dichalcogenides PN-Junctions

Transition metal dichalcogenides (TMDs) are layered semiconductors with indirect band gaps comparable to Si. These compounds can be grown in large area, while their gap(s) can be tuned by changing their chemical composition or by applying a gate voltage. The experimental evidence collected so far points toward a strong interaction with light, which contrasts with the small photovoltaic efficiencies η ≤ 1% extracted from bulk crystals or exfoliated monolayers. Here, we evaluate the potential of these compounds by studying the photovoltaic response of electrostatically generated PN-junctions composed of approximately 10 atomic layers of MoSe2 stacked onto the dielectric h-BN. In addition to ideal diode-like response, we find that these junctions can yield, under AM-1.5 illumination, photovoltaic efficiencies η exceeding 14%, with fill factors of ~70%. Given the available strategies for increasing η such as gap tuning, improving the quality of the electrical contacts, or the fabrication of tandem cells, our study suggests a remarkable potential for photovoltaic applications based on TMDs.

Electronic properties of unstrained unrelaxed narrow gap InAs x Sb1−x alloys

The electronic properties of unstrained unrelaxed InAs x Sb1−x alloys have been determined in a wide range of alloy compositions using IR magnetospectroscopy, magnetotransport and IR photoluminescence. All studied alloys have n-type background doping with electron concentration decreasing with the Sb content. The composition dependence of the background doping concentration follows an empirical exponential law in a wide range of compositions. Both bandgap and electron effective mass dependence on alloy composition exhibit negative bowing reaching lowest values at x = 0.63: E g = 0.10 eV, m* = 0.0082 m 0 at 4.2 K. The bowing coefficient of 0.038 m 0 obtained for the electron effective mass is in good agreement with that obtained from the Kane model.

Cyclotron resonance of single valley Dirac fermions in a gapless HgTe quantum well

We report on Landau level spectroscopy studies of two HgTe quantum wells (QWs) near or at the critical well thickness, where the band gap vanishes. In magnetic fields up to

Determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer quantum dots via spectral analysis of optical signature of the Aharanov-Bohm excitons

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Spin–phonon couplings in transition metal complexes with slow magnetic relaxation

=16T, oriented perpendicular to the QW plane, we observe a

Properties of novel metamorphic III-V materials with ultra-low bandgaps (Conference Presentation)

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Decoherence in semiconductor nanostructures with type-II band alignment: All-optical measurements using Aharonov-Bohm excitons

dependence for the energy of the dominant cyclotron resonance (CR) transition characteristic of two-dimensional Dirac fermions. The dominant CR line exhibits either a single or double absorption lineshape for the gapless or gapped QW. Using an effective Dirac model, we deduce the band velocity of single valley Dirac fermions in gapless HgTe quantum wells,

Optical anisotropy in type-II ZnTe/ZnSe submonolayer quantum dots

v_F=6.4 \times10^5