Novel muonium centers—magnetic polarons—in magnetic semiconductors

Storchak, Vyacheslav G.; Parfenov, Oleg E.; Brewer, J. H.; Russo, Peter L.; Stubbs, Scott L.; Lichti, R.L.; Eshchenko, D. G.; Morenzoni, E.; Зломанов, В. П.; Vinokurov, A. A.; Бамбуров, В. Г.

doi:10.1016/j.physb.2008.11.141

Cited by 16 publications

(21 citation statements)

References 13 publications

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“…This technique has recently been demonstrated in the related EuS host 17 as well as in other magnetic semiconductors 18 following muon spin relaxation ͑ + SR͒ ͑Ref. 19͒ experiments in insulating 20,21 and semiconducting 22-25 media, which have shown that one of the excess electrons generated in the track can be captured by the muon to form a muonium ͑Muϵ + e − ͒ atom.…”

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confidence: 99%

Magnetic polaron bound to the positive muon in SmS: Exchange-driven formation of a mixed-valence state

et al. 2009

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Muon spin-rotation experiments ͑supported by magnetization measurements͒ have been carried out in the canonical 4f mixed-valence narrow-gap semiconductor SmS from 10 to 900 K in magnetic fields up to 3.5 T. A bound state of an electron around a positive muon is found to form up to about 800 K. This state is a magnetic polaron: the electron wave function is confined within R Ϸ 0.5 nm ͑the first two coordination spheres͒ due to its exchange interaction with Sm magnetic moments. As such, it may serve as a model system for the hypothetical bound state suggested to account for a transition from divalent Sm 2+ to trivalent Sm 3+ , which is invoked to explain the transformation of SmS from a paramagnetic insulator into a magnetic metal at high pressure.The problem of spin and charge fluctuations close to a magnetic instability in mixed-valence ͑MV͒ systems has attracted considerable attention ͑see, e.g., Ref. 1 and references therein͒. In strongly correlated 4f electron systems, the class of MV materials known as narrow-gap semiconductors or Kondo insulators ͑SmB 6 , YbB 12 , TmSe, etc.͒ supports hybridization between localized 4f states and itinerant 5d-6s states causing instabilities in charge and magnetic configurations. These materials have been studied for almost four decades 2 but the mechanism of the valence fluctuation remains mysterious. Among them, the canonical MV system SmS makes an ideal material in which to study charge fluctuations in the vicinity of a quantum critical point because of the relative ease with which the Sm ion changes its ion charge state. Although a consensus has been reached that the remarkable properties of SmS may be understood in terms of a Sm 2+ ͑4f 6 ; J =0͒ → Sm 3+ ͑4f 5 ; J =5/ 2͒ + e͑d , s͒ transition, 3 the mechanism of the electron capture/release remains a subject of current interest.Unlike classical magnetic semiconductors ͑Eu chalcogenides, magnetic spinels, etc.͒ which experience metalinsulator transitions ͑MIT͒ close to the magnetic ordering temperature 4,5 at ambient pressure SmS remains a paramagnetic semiconductor within a NaCl crystal structure down to low temperature. 2 As the pressure is increased, SmS exhibits two successive phase transitions. First it undergoes an isostructural transition at the remarkably low pressure of p BG Ϸ 0.65 GPa ͑at room temperature͒, 6 involving a valence change from a Sm 2+ to a homogeneous mixed-valent ͑2.6-2.8͒ state. 2 This first-order phase transition is characterized by a huge volume collapse of up to 15% accompanied by a color change from black to golden. 7 In the black phase, the Fermi level E F falls into a gap between a 4f 6 level and an unoccupied 5d band, thus making Sm 2+ S 2− a nonmagnetic narrow-gap semiconductor. 8 In the golden phase, the 4f 6 level may be pushed into the conduction band, 8,9 resulting in a mix of divalent 4f 6 and trivalent 4f 5 configurations. In this phase the high-temperature resistivity is metallic but at low temperature the ground state is a strongly correlated semiconductor as long as the pressure i...

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confidence: 99%

Magnetic polaron bound to the positive muon in SmS: Exchange-driven formation of a mixed-valence state

et al. 2009

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“…5͒ as well as in other magnetic semiconductors. 6 In an AF in the BMP thus formed, the increase in the electron kinetic energy due to localization is expected to be compensated by the on-site exchange interaction IS / 2 of the electron with local spin S combined with Coulomb interaction with the muon as opposed to the energy NJS 2 required to flip N local spins S with the effective exchange energy J of an AF to produce a FM "droplet" within the radius R so that the change in the free energy,…”

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confidence: 99%

Electron localization into a bound spin polaron in the quasi-one-dimensionalS=12antiferromagnetLiCu2<

et al. 2009

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Muon spin rotation experiments in zero magnetic field and magnetization measurements have been carried out in a single crystal of spin S =1/ 2 double chain cuprate LiCu 2 O 2 over a temperature range of 2-300 K. In the antiferromagnetic state we find a bound state of an electron around the muon-the magnetic polaron-with the electron wave function confined within R = 0.55Ϯ 0.05 nm. Electron localization in this form persists up to 31 K, well above T N = 24.7 K.

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“…84 to suggest two magnetically inequivalent sites occupied by the positive muon in UGe 2 . Although such an approach constitutes the conventional assignment of multiple signals in a magnetically ordered state, fast spin fluctuations make this interpretation irrelevant in the paramagnetic phase 59,60,[62][63][64][65][66] , while possible Knight shifts from the conduction electrons are typically at least 2-3 orders of magnitude smaller than the characteristic splittings detected in this experiment 74 . Moreover, the two peaks do not follow the temperature dependence of the magnetization, which clearly indicates that the muon does not stay "bare" and act as local magnetometer.…”

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confidence: 83%

“…In a PM or metallic (or both) environment, the strong pair exchange interaction of the bound electron with itinerant spins (spin exchange 74,78 ) would result in rapid spin fluctuations of this electron, averaging the hyperfine interaction to zerowhich, in turn, would result in a collapse of the doublet into a single line at ν µ (see Ref. 86 for details), if the local FM ordering mediated by this electron did not hold the electron's spin orientation "locked" 59,60,[62][63][64][65][66]85,86 . In metals, however, even the protective local FM environment of a SP does not ensure observation of the doublet unless the SP spin (S) is decoupled from its magnetic environment 59,66,86 .…”

Section: Figurementioning

confidence: 99%

Spin-polaron band in the ferromagnetic heavy-fermion superconductor UGe₂

Storchak¹,

Brewer²,

Eshchenko³

et al. 2014

J. Phys.: Conf. Ser.

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In strongly correlated materials, cooperative behaviour of the electrons causes a variety of quantum ordered states that may, in some cases, coexist 1-3 . It has long been believed, however, that such coexistence among ferromagnetic ordering, superconductivity and heavy-fermion behaviour is impossible, as the first supports parallel spin alignment while the conventional understanding of the latter two phenomena assumes spin-singlet or antiparallel spins. This understanding has recently been challenged by an increasing number of observations in uranium intermetallic systems (UGe 2 , URhGe, UIr and UCoGe) 4-7 in which superconductivity is found within a ferromagnetic state and, more fundamentally, both ordering phenomena are exhibited by the same set of comparatively heavy 5f electrons. Since the coexistence of superconductivity and ferromagnetism is at odds with the standard theory of phonon-mediated spin-singlet superconductivity, it requires an alternative pairing mechanism, in which electrons are bound into spin-triplet pairs by spin fluctuations 8,9 . Within the heavy-fermion scenario, this alternative mechanism necessarily assumes that the magnetism has a band character and that said band forms from heavy quasiparticles composed of f electrons. This band is expected to be responsible for all three remarkable phenomenaheavy-fermion behaviour, ferromagnetism and superconductivity although its nature and the nature of those heavy quasiparticles still remains unclear. Here we report spectroscopic evidence (from high-field muon spin rotation measurements) for the formation in UGe 2 of subnanometer-sized spin polarons whose dynamics we follow into the paramagnetic and ferromagnetic phases. These spin polarons behave as heavy carriers and thus may serve as heavy quasiparticles made of 5f electrons; once coherence is established, they form a narrow spin-polaron band which thus provides a natural reconciliation of itinerant ferromagnetism with spin-triplet superconductivity and heavy-fermion behaviour.Within the BCS theory of superconductivity (SC), it became clear long ago 10 that pairing of electrons in the spin-singlet state is effectively destroyed by an exchange mechanism arising from strong Coulomb interactions between the valence electrons. In a ferromagnetically (FM) ordered state, this exchange interaction tends to align the spins of electrons within a Cooper pair in parallel, thereby effectively preventing the pairing. Likewise, within the standard heavy-fermion (HF) approach, the Kondo effect

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Novel muonium centers—magnetic polarons—in magnetic semiconductors

Cited by 16 publications

References 13 publications

Magnetic polaron bound to the positive muon in SmS: Exchange-driven formation of a mixed-valence state

Magnetic polaron bound to the positive muon in SmS: Exchange-driven formation of a mixed-valence state

Electron localization into a bound spin polaron in the quasi-one-dimensionalS=12antiferromagnetLiCu2<

Spin-polaron band in the ferromagnetic heavy-fermion superconductor UGe₂

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Novel muonium centers—magnetic polarons—in magnetic semiconductors

Cited by 16 publications

References 13 publications

Magnetic polaron bound to the positive muon in SmS: Exchange-driven formation of a mixed-valence state

Magnetic polaron bound to the positive muon in SmS: Exchange-driven formation of a mixed-valence state

Electron localization into a bound spin polaron in the quasi-one-dimensionalS=12antiferromagnetLiCu2<

Spin-polaron band in the ferromagnetic heavy-fermion superconductor UGe2

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Spin-polaron band in the ferromagnetic heavy-fermion superconductor UGe₂