Point Defects, Ferromagnetism, and Transport in Calcium Hexaboride

Hybridization between conduction electrons and the strongly interacting f-electrons in rare earth or actinide compounds may result in new states of matter. Depending on the exact location of the concomitant hybridization gap with respect to the Fermi energy, a heavy fermion or an insulating ground state ensues. To study this entanglement locally, we conducted scanning tunneling microscopy and spectroscopy (STS) measurements on the "Kondo insulator" SmB 6 . The vast majority of surface areas investigated were reconstructed, but infrequently, patches of varying sizes of nonreconstructed Smor B-terminated surfaces also were found. On the smallest patches, clear indications for the hybridization gap with logarithmic temperature dependence (as expected for a Kondo system) and for intermultiplet transitions were observed. On nonreconstructed surface areas large enough for coherent cotunneling, we were able to observe clear-cut Fano resonances. Our locally resolved STS indicated considerable finite conductance on all surfaces independent of their structure, not proving but leaving open the possibility of the existence of a topologically protected surface state.M aterials with strong electron correlations continue to draw enormous attention, not only because they may give rise to fundamentally new states of matter or new phenomena but also because of the hope for advanced technological applications. Heavy fermion (HF) materials, i.e., intermetallics of certain rare earths (REs), such as Ce, Sm, and Yb, are model systems to study strong electronic correlations (1). Here, the RE-derived localized 4f states are covalently mixed with the conduction-band states and, thus, acquire a finite lifetime. The associated decay rate in relation to the energy of the localized 4f state corresponds to the valency. In an Sm-based HF system, the valence lies between 3+ (4f 5 ) and 2+ (4f 6 ), which implies a considerable amount of charge fluctuations. This usually is referred to as intermediate valence (2). In addition to the abovementioned mixing of 4f states and the conduction band, which is well described within the framework of one-electron models, a manybody interaction is operating between the 4f and conduction electrons. This "Kondo effect" (3) eventually leads to a screening of the local moments as a result of particle-hole excitations that are manifested by a narrow Abrikosov-Suhl, or Kondo, resonance at E F , the width of which is given by the single-ion Kondo temperature T K .Because of the periodic arrangement of REs in an HF intermetallic, the Kondo resonances form a weakly dispersive HF or "coherent 4f−" band, resulting in a heavy Fermi liquid state well below T K . The band interaction between the renormalized 4f and the conduction band generates a so-called hybridization gap, which opens at around T K . Under certain conditions, E F may reside inside this gap, characterizing a so-called Kondo insulator (4).SmB 6 is such a Kondo insulator, with a valence ν ∼ 2:6 (5), ν being slightly temperature dependent (6, 7). A sharp de...

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“…Assuming cleavage along the {001} plane through breaking interoctahedral B-B bonds (40), Fig. 2A exhibits an Sm-terminated surface.…”

Section: Resultsmentioning

confidence: 99%

Hybridization gap and Fano resonance in SmB₆

Rößler¹,

Jang

Kim

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

127

160

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“…Moreover, our scenario is also consistent with the experimental results that deficiency in Ca sites [28] or doping of divalent elements like Ba [1] or Sr [29] induces ferromagnetism. It is also confirmed numerically by a supercell approach that imperfections and surfaces can induce a local moment [30]. It is hard to explain them within the spin-doping scenario [3].…”

mentioning

confidence: 99%

Theory of Ferromagnetism inCa1−xLaxB6
Murakami
¹
,
Shindou
²
,
Nagaosa
³

et al. 2002
Phys. Rev. Lett.
16
1
23
0
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Novel ferromagnetism in Ca1−xLaxB6 is studied in terms of the Ginzburg-Landau theory for excitonic order parameters, taking into account symmetry of the wavefunctions. We found that the minima of the free energy break both inversion and time-reversal symmetries, while the product of these two remains preserved. This explains various novelties of the ferromagnetism and predicts a number of magnetic properties, including the magnetoelectric effect, which can be tested experimentally.Novel ferromagnetism in Ca 1−x La x B 6 [1] has been the subject of extensive studies due to its high Curie temperature (∼ 600K) in spite of a small moment (0.07µ B /La) and lack of partially filled d-or f -bands. However, its mechanism still remains controversial. Ceperly suggested that this is a first example of the ferromagnetic phase of a dilute electron gas [2]. An alternative explanation [3,4] is based on the excitonic state [5] of the parent material CaB 6 . The parent compound CaB 6 has a cubic structure, and some band structure calculations [6,7] predict a small overlap of the conduction and valence bands at the X-points. Matrix elements of the dipole moments vanish between these states at the X-points; the dielectric constant is not enhanced even when the band gap collapses. This makes the excitonic insulator a plausible candidate for the ground state of CaB 6 [3]. By La-doping the extra electrons are doped into this excitonic insulator, and it has been found in the mean-field approximation that these electrons are perfectly spin-polarized [3]. Nevertheless, there is so far no conclusive explanation for the magnitude of the moment, much smaller than the electronic moment doped by La. Furthermore, the correlation between the La-doping concentration and the ferromagnetic moments are questioned by recent experiments [8]. Several experiments suggest that the ferromagnetism is not a bulk phenomenon, and occurs only in the thin film sample [9], or near the surface as evidenced by an electron spin resonance (ESR) experiment [10].In this letter we study in depth the symmetry properties of the excitonic state in CaB 6 in terms of the Ginzburg-Landau (GL) theory, and propose a possible scenario for the novel ferromagnetism. We classify possible states in terms of the magnetic point groups. The idea is that CaB 6 is a triplet excitonic insulator [3] with broken time-reversal (R) and inversion (I) symmetries, while their product RI is kept intact [3]. This means that CaB 6 is an antiferromagnet (AF), and the ferromagnetism is induced by the magneto-electric (ME) effect, and the La-doping and/or the surface works mainly as a source of the electric field. We emphasize that the present theory of ferromagnetism is different from those in [3,4], though all these theories are based on exciton condensation.This scenario of exciton condensation is justified when the band structure has a small overlap/gap at the X points. It is, however, still in controversy whether it is the case. Contrary to the LDA calculation [6,7], more recent LDA+GW calcula...

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“…18 We suggest that these states overlap to form an impurity band, which somehow mediates a long-range ferromagnetic interaction between the magnetic moments associated with molecular orbitals in the vicinity of the defects. The appearance of magnetic moments on anion defects was proposed by Monnier and Delley, 19 who ascribed the ferromagnetism in CaB 6 to anion vacancies (F 0 centers), although the exchange mechanism was not explained. A neutral B 6 vacancy was found to create a moment of 2.4 B distributed over the six neighbouring boron octahedra.…”

mentioning

confidence: 99%

Magnetism in thin films of CaB6 and SrB6

Dorneles

Venkatesan

Moliner

et al. 2004

Applied Physics Letters

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Thin films of disordered hexaborides CaB 6 and SrB 6 deposited by pulsed-laser deposition on MgO (100) or Al 2 O 3 (001) substrates are ferromagnetic. A typical room-temperature moment per unit area of substrate is 350 B nm −2 , with the largest values being found for CaB 6 on Al 2 O 3 . Lattice defects are the likely origin of the exotic, high-temperature magnetism. The moment, which is present in films as thin as 12 nm, appears to reside in an interface layer whose polarization is approximately 0.4 Tesla.

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Point Defects, Ferromagnetism, and Transport in Calcium Hexaboride

Cited by 117 publications

References 23 publications

Hybridization gap and Fano resonance in SmB₆

Hybridization gap and Fano resonance in SmB₆

Theory of Ferromagnetism inCa1−xLaxB6
Murakami
¹
,
Shindou
²
,
Nagaosa
³

et al. 2002
Phys. Rev. Lett.
16
1
23
0
View full text Add to dashboard Cite

Magnetism in thin films of CaB6 and SrB6

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Point Defects, Ferromagnetism, and Transport in Calcium Hexaboride

Cited by 117 publications

References 23 publications

Hybridization gap and Fano resonance in SmB6

Hybridization gap and Fano resonance in SmB6

Theory of Ferromagnetism inCa1−xLaxB6Murakami1, Shindou2, Nagaosa3 et al. 2002Phys. Rev. Lett.161230View full textAdd to dashboardCite

Magnetism in thin films of CaB6 and SrB6

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Hybridization gap and Fano resonance in SmB₆

Hybridization gap and Fano resonance in SmB₆

Theory of Ferromagnetism inCa1−xLaxB6
Murakami
¹
,
Shindou
²
,
Nagaosa
³

et al. 2002
Phys. Rev. Lett.
16
1
23
0
View full text Add to dashboard Cite