The Spin Excitation Spectrum in Quantum Hall Systems: Insights from Light Scattering Experiments

Gallais, Yann; Kirschenmann, Thomas; Yan, Jun; Pinczuk, A.; Pfeiffer, L. N.; West, K. W.

doi:10.1142/s0217979207042653

Int. J. Mod. Phys. B

2007

DOI: 10.1142/s0217979207042653

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The Spin Excitation Spectrum in Quantum Hall Systems: Insights from Light Scattering Experiments

Yann Gallais

Thomas Kirschenmann

Jun Yan

et al.

Abstract: Collective spin excitations in quantum Hall systems are studied via inelastic light scattering. In the fractional quantum Hall effect regime, composite fermion spin excitations are observed in the range 1/3< ν <2/5. They reveal a transition from free to strongly interacting composite fermions. At ν=1, a shift of the spin-wave energy at finite wavevector from the bare Zeeman energy is observed. It allows us to evaluate the spin-stiffness of the quantum Hall ferromagnet.

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“…At ν=1 the spectrum consists of a single sharp (FWHM∼30 µeV) peak which is identified as the ferromagnetic spin wave (SW) at the wave vector q. Its energy is blue-shifted from E z due to the combined effects of the finite wave vector transfer q and the large spin-stiffness of the ν=1 quantum Hall ferromagnet [22].…”

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Soft Spin Wave nearν=1: Evidence for a Magnetic Instability in Skyrmion Systems

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The ground state of the two dimensional electron gas near ν=1 is investigated by inelastic light scattering measurements carried down to very low temperatures. Away from ν=1, the ferromagnetic spin wave collapses and a new low-energy spin wave emerges below the Zeeman gap. The emergent spin wave shows soft behavior as its energy increases with temperature and reaches the Zeeman energy for temperatures above 2 K. The observed softening indicates an instability of the two dimensional electron gas towards a magnetic order that breaks spin rotational symmetry. We discuss our findings in light of the possible existence of a Skyrme crystal.In condensed matter physics, the competition between distinct correlated electron ground states follows from a delicate interplay between dimensionality, strength of Coulomb interactions, and disorder. Correlated states may possess long range spin and/or charge order that breaks a symmetry that, in turn, has profound consequences on the nature of the low-energy excitation spectrum [1,2]. One well-known example of the relationship between broken-symmetry and elementary excitations is the isotropic ferromagnet. The broken rotational spin symmetry leads to appearance of gapless spin wave excitations, Goldstone modes, that are fluctuations of the spin density around the magnetization direction.The ground state of the two-dimensional (2D) electron gas at Landau level filling factor ν=1 is a special kind of itinerant ferromagnet, the quantum Hall ferromagnet, where all electrons occupy the lowest orbital Landau level and their spins are aligned along the external magnetic field. The quantum Hall ferromagnet supports collective excitations similar to ferromagnetic spin-waves with a gap given by the bare Zeeman energy E z [3,4]. The quantum Hall ferromagnet also has spin texture excitations built from Skyrmions [5,6], a concept used to describe the emergence of nuclear particles in the context of field theories of nuclear matter [7]. Skyrmions are topological objects which smoothly distort the ferromagnetic order in a vortex-like configuration. Each individual Skyrmion involves several flipped spins and is therefore not favored by the Zeeman energy. On the other hand because the exchange energy is large in quantum Hall systems, and it prefers locally aligned spins, Skyrmions are cheaper than single spin-flips for sufficiently low E z . The relevance of Skyrmions has been demonstrated by a wide range of experiments probing the spin polarization of the 2DES [8,9,10,11].Near ν=1 the interaction between Skyrmions may lock orientations of spin in the XY plane to favor the formation of a Skyrme crystal of electron spin orientation that breaks spin rotational symmetry about the magnetic field axis [12]. As a consequence of the additional symmetry breaking, the 2D electron system supports spin waves which, contrary to the ferromagnetic spin wave, remain gapless in the presence of the magnetic field [13,14,15]. Both a jump in the specific heat and the very short T 1 observed away from ν=1 were inter...

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Soft Spin Wave nearν=1: Evidence for a Magnetic Instability in Skyrmion Systems

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Inelastic light scattering measurements of spin excitations in Skyrmion systems

Gallais

Pinczuk

Pfeiffer³

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Physica E: Low-dimensional Systems and Nanostructures

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Spin Texture and Magnetoroton Excitations atν=1/3

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Neutral spin texture (ST) excitations at nu=1/3 are directly observed for the first time by resonant inelastic light scattering. They are determined to involve two simultaneous spin flips. At low magnetic fields, the ST energy is below that of the magnetoroton minimum. With increasing in-plane magnetic field these mode energies cross at a critical ratio of the Zeeman and Coulomb energies of eta(c)=0.020+/-0.001. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magnetoroton modes collapse. The temperature dependence is interpreted in terms of a competition between coexisting phases supporting different excitations. We consider the role of the ST excitations in activated transport at nu=1/3.

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The Spin Excitation Spectrum in Quantum Hall Systems: Insights from Light Scattering Experiments

Cited by 3 publications

References 26 publications

Soft Spin Wave nearν=1: Evidence for a Magnetic Instability in Skyrmion Systems

Soft Spin Wave nearν=1: Evidence for a Magnetic Instability in Skyrmion Systems

Inelastic light scattering measurements of spin excitations in Skyrmion systems

Spin Texture and Magnetoroton Excitations atν=1/3

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