Direct observation of excitonic instability in Ta2NiSe5

AbstractCoulomb attraction between electrons and holes in a narrow-gap semiconductor or a semimetal is predicted to lead to an elusive phase of matter dubbed excitonic insulator. However, direct observation of such electronic instability remains extremely rare. Here, we report the observation of incipient divergence in the static excitonic susceptibility of the candidate material Ta2NiSe5 using Raman spectroscopy. Critical fluctuations of the excitonic order parameter give rise to quasi-elastic scattering of B2g symmetry, whose intensity grows inversely with temperature toward the Weiss temperature of TW ≈ 237 K, which is arrested by a structural phase transition driven by an acoustic phonon of the same symmetry at TC = 325 K. Concurrently, a B2g optical phonon becomes heavily damped to the extent that its trace is almost invisible around TC, which manifests a strong electron-phonon coupling that has obscured the identification of the low-temperature phase as an excitonic insulator for more than a decade. Our results unambiguously reveal the electronic origin of the phase transition.

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Structural phase transition and superconductivity hierarchy in 1T-TaS2 under pressure up to 100 GPa

npj Quantum Materials ◽

10.1038/s41535-021-00320-x ◽

2021 ◽

Vol 6 (1) ◽

Author(s):

Qing Dong ◽

Quanjun Li ◽

Shujia Li ◽

Xuhan Shi ◽

Shifeng Niu ◽

...

Keyword(s):

Structural Phase ◽

Transition Metal Dichalcogenides ◽

Phonon Coupling ◽

Structural Phase Transitions ◽

Strong Electron ◽

Electron Phonon Coupling ◽

Increasing Trend ◽

Metal Dichalcogenides ◽

Dome Shape ◽

Enhanced Superconductivity

AbstractThe adoption of high pressure not only reinforces the comprehension of the structure and exotic electronic states of transition metal dichalcogenides (TMDs) but also promotes the discovery of intriguing phenomena. Here, 1T-TaS2 was investigated up to 100 GPa, and re-enhanced superconductivity was found with structural phase transitions. The discovered I4/mmm TaS2 presents strong electron–phonon coupling, revealing a good superconductivity of the nonlayered structure. The P–T phase diagram shows a dome shape centered at ~20 GPa, which is attributed to the distortion of the 1T structure. Accompanied by the transition to nonlayered structure above 44.5 GPa, the superconducting critical temperature shows an increasing trend and reaches ~7 K at the highest studied pressure, presenting superior superconductivity compared to the original layered structure. It is unexpected that the pressure-induced re-enhanced superconductivity was observed in TMDs, and the transition from a superconductor with complicated electron-pairing mechanism to a phonon-mediated superconductor would expand the field of pressure-modified superconductivity.

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Phase transition and proton ordering at 50 K in 3-(pyridin-4-yl)pentane-2,4-dione

Acta Crystallographica Section B Structural Science Crystal Engineering and Materials ◽

10.1107/s2052520617015591 ◽

2017 ◽

Vol 73 (6) ◽

pp. 1172-1178 ◽

Cited By ~ 4

Author(s):

Khai-Nghi Truong ◽

Carina Merkens ◽

Martin Meven ◽

Björn Faßbänder ◽

Richard Dronskowski ◽

...

Keyword(s):

Phase Transition ◽

Low Temperature ◽

Single Crystal ◽

Structural Phase ◽

Pyridyl Ring ◽

Temperature Phase ◽

Crystallographic Axis ◽

Temperature Phase Transition ◽

Intramolecular Hydrogen ◽

Low Temperature Phase

Single-crystal neutron diffraction experiments at 100 and 2.5 K have been performed to determine the structure of 3-(pyridin-4-yl)pentane-2,4-dione (HacacPy) with respect to its protonation pattern and to monitor a low-temperature phase transition. Solid HacacPy exists as the enol tautomer with a short intramolecular hydrogen bond. At 100 K, its donor···acceptor distance is 2.450 (8) Å and the compound adopts space group C2/c, with the N and para-C atoms of the pyridyl ring and the central C of the acetylacetone substituent on the twofold crystallographic axis. As a consequence of the axial symmetry, the bridging hydrogen is disordered over two symmetrically equivalent positions, and the carbon–oxygen bond distances adopt intermediate values between single and double bonds. Upon cooling, a structural phase transition to the t 2 subgroup P\bar 1 occurs; the resulting twins show an ordered acetylacetone moiety. The phase transition is fully reversible but associated with an appreciable hysteresis in the large single crystal under study: transition to the low-temperature phase requires several hours at 2.5 K and heating to 80 K is required to revert the transformation. No significant hysteresis is observed in a powder sample, in agreement with the second-order nature of the phase transition.

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Structural distortion and electronic states of Rb doped WO3 by X-ray absorption spectroscopy

RSC Advances ◽

10.1039/c6ra21777j ◽

2016 ◽

Vol 6 (109) ◽

pp. 107871-107877 ◽

Cited By ~ 3

Author(s):

Y. C. Wang ◽

C. H. Hsu ◽

Y. Y. Hsu ◽

C. C. Chang ◽

C. L. Dong ◽

...

Keyword(s):

Structural Phase ◽

Electronic States ◽

Phonon Coupling ◽

Structural Phase Transitions ◽

Superconducting Properties ◽

Strong Electron ◽

X Ray ◽

Electron Phonon Coupling ◽

Complex Structural ◽

X Ray Absorption

Rubidium tungsten bronzes (RbxWO3) have recently attracted much attention due to their intriguing phenomena, such as complex structural phase transitions, strong electron–phonon coupling, and superconducting properties.

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Structural phase transition and hydrogen ordering of TlH2PO4 at low temperature

Acta Crystallographica Section B Structural Science ◽

10.1107/s0108768106018222 ◽

2006 ◽

Vol 62 (5) ◽

pp. 719-728 ◽

Cited By ~ 16

Author(s):

I. H. Oh ◽

M. Merz ◽

S. Mattauch ◽

G. Heger

Keyword(s):

Crystal Structure ◽

Phase Transition ◽

Low Temperature ◽

Structural Phase ◽

Crystal Structure Analysis ◽

Temperature Phase ◽

Structure Model ◽

Triclinic Space Group ◽

Space Groups ◽

Low Temperature Phase

The crystal structure of TlH2PO4 (TDP) has been studied at low temperature. The lattice parameters were derived from high-resolution X-ray powder diffraction in the temperature range between 8 and 300 K. A detailed crystal structure analysis of the antiferroelectric low-temperature phase TDP-III has been performed based on neutron diffraction data measured at 210 K on a twinned crystal consisting of two domain states. The structure model in the triclinic space group P\bar 1 is characterized by a complete ordering of all the H atoms in the asymmetric O—H...O hydrogen bonds. The phase transition from the ferroelastic TDP-II to the antiferroelectric TDP-III phase at 229.5 ± 0.5 K is only slightly of first order and shows no detectable hysteresis effects. Its mechanism is driven by the hydrogen ordering between the partially ordered TDP-II state and the completely ordered TDP-III state. The polymorphism of TDP and the fully deuterated TlD2PO4 (DTDP) is presented in the form of group–subgroup relations between the different space groups.

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Strong Electron–Phonon Coupling in the Excitonic Insulator Ta2NiSe5

Inorganic Chemistry ◽

10.1021/acs.inorgchem.9b00432 ◽

2019 ◽

Vol 58 (14) ◽

pp. 9036-9042 ◽

Cited By ~ 5

Author(s):

Jian Yan ◽

Ruichun Xiao ◽

Xuan Luo ◽

Hongyan Lv ◽

Ranran Zhang ◽

...

Keyword(s):

Phonon Coupling ◽

Strong Electron ◽

Excitonic Insulator ◽

Electron Phonon Coupling

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Monoclinic-to-orthorhombic phase transition of the hexamethylenetetramine–2-methylbenzoic acid (1/2) cocrystal with temperature-dependent dynamic molecular disorder

Acta Crystallographica Section C Structural Chemistry ◽

10.1107/s2053229616016831 ◽

2016 ◽

Vol 72 (12) ◽

pp. 971-980 ◽

Cited By ~ 1

Author(s):

Tze Shyang Chia ◽

Ching Kheng Quah

Keyword(s):

Phase Transition ◽

High Temperature ◽

Low Temperature ◽

Structural Phase ◽

Point Group ◽

High Temperature Phase ◽

Temperature Phase ◽

Temperature Dependent ◽

Space Group ◽

Low Temperature Phase

As a function of temperature, the hexamethylenetetramine–2-methylbenzoic acid (1/2) cocrystal, C6H12N4·2C8H8O2, undergoes a reversible structural phase transition. The orthorhombic high-temperature phase in the space groupPccnhas been studied in the temperature range between 165 and 300 K. At 164 K, at2phase transition to the monoclinic subgroupP21/cspace group occurs; the resulting twinned low-temperature phase was investigated in the temperature range between 164 and 100 K. The domains in the pseudomerohedral twin are related by a twofold rotation corresponding to the matrix (100/0-10/00-1. Systematic absence violations represent a sensitive criterium for the decision about the correct space-group assignment at each temperature. The fractional volume contributions of the minor twin domain in the low-temperature phase increases in the order 0.259 (2) → 0.318 (2) → 0.336 (2) → 0.341 (3) as the temperature increases in the order 150 → 160 → 163 → 164 K. The transformation occurs between the nonpolar point groupmmmand the nonpolar point group 2/m, and corresponds to a ferroelastic transition or to at2structural phase transition. The asymmetric unit of the low-temperature phase consists of two hexamethylenetetramine molecules and four molecules of 2-methylbenzoic acid; it is smaller by a factor of 2 in the high-temperature phase and contains two half molecules of hexamethylenetetramine, which sit across twofold axes, and two molecules of the organic acid. In both phases, the hexamethylenetetramine residue and two benzoic acid molecules form a three-molecule aggregate; the low-temperature phase contains two of these aggregates in general positions, whereas they are situated on a crystallographic twofold axis in the high-temperature phase. In both phases, one of these three-molecule aggregates is disordered. For this disordered unit, the ratio between the major and minor conformer increases upon cooling from 0.567 (7):0.433 (7) at 170 Kvia0.674 (6):0.326 (6) and 0.808 (5):0.192 (5) at 160 K to 0.803 (6):0.197 (6) and 0.900 (4):0.100 (4) at 150 K, indicating temperature-dependent dynamic molecular disorder. Even upon further cooling to 100 K, the disorder is retained in principle, albeit with very low site occupancies for the minor conformer.

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The square-planar to flattened-tetrahedral CuX42−(X= Cl, Br) structural phase transition in 1,2,6-trimethylpyridinium salts

Acta Crystallographica Section B Structural Science Crystal Engineering and Materials ◽

10.1107/s205252061402664x ◽

2015 ◽

Vol 71 (1) ◽

pp. 48-60 ◽

Cited By ~ 7

Author(s):

Annette Kelley ◽

Sowjanya Nalla ◽

Marcus R. Bond

Keyword(s):

Phase Transition ◽

Phase Transformation ◽

Hydrogen Bonding ◽

Low Temperature ◽

Structural Phase ◽

Temperature Phase ◽

Tetrahedral Geometry ◽

Square Planar ◽

Higher Temperature ◽

Low Temperature Phase

The abrupt green-to-yellow thermochromism observed for certainA2CuCl4compounds has long been attributed to the presence of strong N—H hydrogen bonding in the low-temperature phase that favors square-planar CuCl42−(green in color) through reduction of ligand–ligand repulsion. Weakening of the hydrogen bonding at higher temperature results in the transformation to (far more common) flattened-tetrahedral geometry – the expected geometry for CuBr42−complexes due to their greater ligand–ligand repulsion. The square-planar to flattened-tetrahedral transitions in (1,2,6-trimethylpyridinium)2CuX4provide the first examples of this phase transformation in the absence of N—H hydrogen bonding and for a CuBr42−complex. These results suggest that the square-planar to flattened-tetrahedral transformation in CuX42−systems may be more common than previously thought.

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