We report on a colossal negative magnetoresistance (MR) in GaAs/AlGaAs quantum well which, at low temperatures, is manifested by a drop of the resistivity by more than an order of magnitude at a magnetic field B ≈ 1 kG. In contrast to MR effects discussed earlier, the MR reported here is not parabolic, even at small B, and persists to much higher in-plane magnetic fields and temperatures. Remarkably, the temperature dependence of the resistivity at B ≈ 1 kG is linear over the entire temperature range studied (from 1 to 30 K) and appears to coincide with the high-temperature limit of the zero-field resistivity, hinting on the important role of acoustic phonons.

Pairing interaction between fermionic particles leads to composite Bosons that condense at low temperature. Such condensate gives rise to long range order and phase coherence in superconductivity, superfluidity, and other exotic states of matter in the quantum limit. In graphene double-layers separated by an ultra-thin insulator, strong interlayer Coulomb interaction introduces electron-hole pairing across the two layers, resulting in a unique superfluid phase of interlayer excitons. In this work, we report a series of emergent fractional quantum Hall ground states in a graphene double-layer structure, which is compared to an expanded composite fermion model with two-component correlation.The ground state hierarchy from bulk conductance measurement and Hall resistance plateau from Coulomb drag measurement provide strong experimental evidence for a sequence of effective integer quantum Hall effect states for the novel two-component composite fermions (CFs), where CFs fill integer number of effective LLs (Λ-level). Most remarkably, a sequence of incompressible states with interlayer correlation are observed at half-filled Λ-levels, which represents a new type of order involving pairing states of CFs that is unique to graphene double-layer structure and beyond the conventional CF model.Within the narrowly dispersing landau levels (LLs) that define the quantum Hall effect (QHE) regime, the kinetic energy is quenched. The resulting electron behaviour is therefore determined almost entirely by minimizing Coulomb repulsion. This results in the series of correlated states appearing at fractional LL filling, known as the fractional quantum Hall effect (FQHE) [1,2]. In double-layer quantum wells consisting of closely spaced parallel 2DEGs, even richer QHE physics emerges. In the small separation limit, the additional layer degree of freedom and interlayer Coulomb interactions lead to a variety of new correlated states that are tunable with interlayer separation and transverse displacement fields.Experimentally observed examples include formation of a superfluid exciton condensate [3][4][5][6][7][8] between electrons in one layer and holes in the other, occurring at total integer LL filling (half filling in each layer) and even denominator FQHE states at 1/2 and 1/4 total filling [9][10][11][12][13]. Theoretical work has identified a host of other possible states at fractional total filling, some of which are expected to be exotic non-Abelian states with topologically non-trivial excitations [14][15][16][17][18][19][20]. However, compared to single layer systems the FQHE in double layers has been less explored experimentally, and many of these states remain unobserved.Here we report measurement of the FQHE in dualgated double-layer graphene (DLG) heterostructures where the active regions consist of two graphene monolayers separated by a layer of hexagonal boron nitride (hBN) (see SI for details of the device structure). Several recent efforts have demonstrated that in DLG, the hBN spacer can be made as thin as 1-2 nm before...

. (2014) Observation of microwave-induced resistance oscillations in a highmobility two-dimensional hole gas in a strained Ge/SiGe quantum well. Physical Review

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We report on the effect of in-plane magnetic field B on stripe phases in higher (N = 2, 3) Landau levels of a high-mobility 2D electron gas. In accord with previous studies, we find that a modest B applied parallel to the native stripes aligns them perpendicular to it. However, upon further increase of B , stripes are reoriented back to their native direction. Remarkably, applying B perpendicular to the native stripes also aligns stripes parallel to it. Thus, regardless of the initial orientation of stripes with respect to B , stripes are ultimately aligned parallel to B . These findings provide evidence for a B -induced symmetry breaking mechanism which challenge current understanding of the role of B and should be taken into account when determining the strength of the native symmetry breaking potential. Finally, our results might indicate nontrivial coupling between the native and external symmetry breaking fields, which has not yet been theoretically considered.Electronic liquid crystal-like phases, termed electron nematics or stripes, are expected to form in a wide variety of condensed matter systems [1][2][3][4][5]. A two-dimensional electron gas in GaAs/AlGaAs hosts the first, and perhaps the most striking, realization of such phases [6][7][8][9][10][11]. Stripes in a two-dimensional electron gas form due to interplay between exchange and direct Coulomb interactions [6,7,10,11] and are manifested by the resistivity minima (maxima) in the easy (hard) transport direction near half-integer filling factors, ν = 9/2, 11/2, 13/2, ... when the system is cooled below T ≈ 0.1 K. With very few exceptions [12][13][14], stripes in GaAs are aligned along 110 direction, but what exactly causes such orientation remains unknown [13,15,16].While the origin of the native symmetry-breaking potential responsible for preferred stripes orientation remains elusive, its magnitude was routinely obtained from experiments employing in-plane magnetic field B which provides an external symmetry-breaking field competing with and overcoming the native one. Our current understanding of B -induced symmetry-breaking potential is based on finite thickness effects [17,18], which favor stripes perpendicular to B , consistent with previous experiments [13,[19][20][21][22][23]]. The same approach successfully explains B -induced stripes in both single-subband [19,20,[24][25][26] and double-subband [27] systems.In this Rapid Communication we re-examine the effect of in-plane magnetic field on quantum Hall stripes in ultrahigh quality GaAs quantum wells. In agreement with early experiments [19][20][21], we find that a B 0.4 applied parallel to the native stripes aligns stripes perpendicular to it. Remarkably, upon further increase of B , stripes are reoriented back to their native direction, i.e. parallel to B . When B is applied perpendicular to the native stripes, we also find that stripes are re-oriented parallel to B . We thus conclude that there exist a new B -induced symmetry-breaking potential which challenge our understanding of the role ...