Tunnelling spectroscopic study of the CDW energy gap in TiSe<sub>2</sub>

Miyahara, Yoichi; Bando, Hiroyuki; Ozaki, Hajime

doi:10.1088/0953-8984/7/13/006

Cited by 9 publications

(10 citation statements)

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“…A third kink in the tunneling spectra at ε 3 ≈ −200 meV is probably associated with the maximum of the second Se 4p band, as suggested by ARPES measurements. Our data seem to be in disagreement with a very high value of the gap of about 0.5 eV at 77 K reported from tunnel junction, with Al counterelectrode, measurements [15] . Although, we see in our spectra other features at higher energies like the kink at +350 meV, we do believe that these features are associated with higher energy bands details.…”

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confidence: 51%

“…The chiral phase is still present in doped samples, close to the critical doping at which the CDW disappears. Despite extensive studies of TiSe 2 , tunneling spectroscopy has only been reported by using tunnel junctions [15] showing a very high value of the CDW gap in disagreement with ARPES and optical spectroscopy [16]. No tunneling spectroscopy has been reported so far for the Cu-doped samples.…”

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confidence: 99%

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Evolution of the charge density wave state in CuxTiSe2

et al. 2012

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We present scanning tunneling microscopy and spectroscopy measurements of the charge-density wave state in 1T -TiSe2, Cu0.05TiSe2 and Cu0.06TiSe2 single crystals. Topography images at 4.2 K reveal that the charge density waves are present in all samples studied, although the amplitude of the charge modulation decreases with the Cu-doping. Moreover, the chiral phase of the charge density wave is preserved also in Cu-doped samples. Tunneling spectroscopy shows that there is only a partial gap in the pure compound, with bands crossing the Fermi surface. In the Cu-doped samples the system becomes more metallic due to the increase of the chemical potential.PACS numbers: 71.45. Lr, 74.55.+v, 72.80.Ga, 73.22.Gk INTRODUCTIONTransition metal dichalcogenides are quasi-twodimensional, highly anisotropic compounds that often show instabilities to charge density wave (CDW) formation at low temperature. The chalcogenide atoms form two parallel layers with the atoms in a hexagonal arrangement. The transition metal atoms exist in between these two layers. In the 1T -type crystal structure the transition metal atoms are octahedrally coordinated. In particular, 1T -TiSe 2 undergoes a CDW phase transition below 200 K with a formation of a commensurate (2a 0 × 2a 0 × 2c 0 ) superlattice [1] that involves a small ionic displacement( 0.08Å), and it is accompanied by a phonon softening [2]. ARPES measurements revealed the band structure, with some of the details that still remain controversial. In the normal state, above the CDW state, TiSe 2 is either a semimetal [3,4] or a semiconductor [5,6] with a small indirect gap. The Se 4p valence band is at the Brillouin zone center Γ while the Ti 3d conduction band forms pockets at the Brillouin zone boundary L. In the CDW state the Se 4p bands become backfolded. The origin of the CDW state remains still a matter of controversy up to date. The CDW transition is not likely to originate from nesting since parallel sheets of the Fermi surface were not detected by ARPES measurements. The proposed scenarios are an indirect Jahn-Teller effect [7], an exciton insulator mechanism [8] or an exciton-phonon driven CDW [9]. Being a material with bands very close to the Fermi level, TiSe 2 is a unique candidate for the excitonic mechanism. Unlike many semiconductors, TiSe 2 has a large number of states close to E F that makes it favorable for collective phenomena to take place. On the other hand, the small total number of carriers yields to a poorly screened Coulomb interaction, so the system is unstable to formation of excitons. However, the softening of the L − 1 mode has been observed in x-ray experiments and it might have some relevance to the CDW formation as well [2]. The discovery of superconductivity upon intercalation of Cu has further attracted the attention to this material [10]. With Cu doping it was found that the CDW transition temperature drops and the superconductivity sets in at a doping x = 0.04 with a highest superconducting transition temperature of 4.15 K occurring at x = 0.08. With ...

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Evolution of the charge density wave state in CuxTiSe2

et al. 2012

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“…4(a) of EP, EE+EP, and with CDW and EE+EP interactions. We include CDW at nesting Q = (1/2,1/2,1/2) as demonstrated experimentally [12], and the mean-field CDW gap is taken to be = 13.4k B T CDW at various dopings, which reasonably reproduces the experimental gap [38]. The self-energy properties and the peak-dip-hump feature in the DOS are all characteristically analogous to MoS 2 .…”

Section: B Tisementioning

confidence: 82%

“…We include CDW at nesting Q=(1/2,1/2,1/2) as demonstrated experimentally, 12 and the mean-field CDW gap is taken to be ∆ = 13.4k B T CDW at various dopings, which reasonably reproduces the experimental gap. 39 The self-energy properties and the peak-dip-hump feature in the DOS are all characteristically analogous to MoS 2 . Moreover, in TiSe 2 the spincomponent dominates the fluctuation strength, and then follows the contributions of charge and phonon terms in order.…”

Section: Parametersmentioning

confidence: 94%

Superconducting dome inMoS2andTiSe2generated by quasiparticle-phonon coupling

Das

Dolui

2015

Phys. Rev. B

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We use a first-principles based self-consistent momentum-resolved density fluctuation (MRDF) model to compute the combined effects of electron-electron and electron-phonon interactions to describe the superconducting dome in the correlated MoS 2 thin flake and TiSe 2 . We find that without including the electron-electron interaction, the electron-phonon coupling and the superconducting transition temperature (T c ) are overestimated in both these materials. However, once the full angular and dynamical fluctuations of the spin and charge density induced quasiparticle self-energy effects are included, the electron-phonon coupling and T c are reduced to the experimental value. With doping, both electronic correlation and electron-phonon coupling grows, and above some doping value, the former becomes so large that it starts to reduce the quasiparticle-phonon coupling constant and T c , creating a superconducting dome, in agreement with experiments.

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“…The two-color pump-probe pulse method [15][16][17] was employed with the cleaved TiSe 2 surface using a microoptical setup as mentioned in Ref. [18].…”

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Chiral symmetry breaking and charge order

Wezel

Littlewood²

2010

Physics

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We discovered the chirality of charge-density waves (CDW) in 1T-TiSe 2 by using STM and timedomain optical polarimetry. We found that the CDW intensity becomes Ia 1 :Ia 2 :Ia 3 ¼ 1:0:7 AE 0:1:0:5 AE 0:1, where Ia i (i ¼ 1; 2; 3) is the amplitude of the tunneling current contributed by the CDWs. There were two states, in which the three intensity peaks of the CDW decrease clockwise and anticlockwise. The chirality in CDW results in the threefold symmetry breaking. Macroscopically, twofold symmetry was indeed observed in optical measurement. We propose the new generalized CDW chirality H CDW q 1 Á ðq 2 Â q 3 Þ, where q i are the CDW q vectors, which is independent of the symmetry of components. The nonzero H CDW-the triple-q vectors do not exist in an identical plane in the reciprocal space-should induce a real-space chirality in CDW system.

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Tunnelling spectroscopic study of the CDW energy gap in TiSe₂

Abstract: The CDW energy gap structure of TiSe2 has been observed by tunnelling spectroscopy and its temperature dependence obtained for 77 K

Cited by 9 publications

References 22 publications

Evolution of the charge density wave state in CuxTiSe2

Evolution of the charge density wave state in CuxTiSe2

Superconducting dome inMoS2andTiSe2generated by quasiparticle-phonon coupling

Chiral symmetry breaking and charge order

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Tunnelling spectroscopic study of the CDW energy gap in TiSe2

Abstract: The CDW energy gap structure of TiSe2 has been observed by tunnelling spectroscopy and its temperature dependence obtained for 77 K

Cited by 9 publications

References 22 publications

Evolution of the charge density wave state in CuxTiSe2

Evolution of the charge density wave state in CuxTiSe2

Superconducting dome inMoS2andTiSe2generated by quasiparticle-phonon coupling

Chiral symmetry breaking and charge order

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Tunnelling spectroscopic study of the CDW energy gap in TiSe₂