Raman spectroscopy of metals, high‐temperature superconductors and related materials under high pressure

Goncharov, Alexander F.; Struzhkin, Viktor V.

doi:10.1002/jrs.1030

Cited by 94 publications

(76 citation statements)

References 137 publications

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“…Fig. 4 shows the measured T c (P) and Raman shift B 1g (P) of B 1g phonon mode obtained by fitting the Raman data in terms of a Green's function approach (38). Note that the measured T c (P) behavior of this material is consistent with other independent measurements (39,40).…”

Section: [3]supporting

confidence: 79%

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Unified picture of the oxygen isotope effect in cuprate superconductors

Chen

Struzhkin

et al. 2007

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

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High-temperature superconductivity in cuprates was discovered almost exactly 20 years ago, but a satisfactory theoretical explanation for this phenomenon is still lacking. The isotope effect has played an important role in establishing electron-phonon interaction as the dominant interaction in conventional superconductors. Here we present a unified picture of the oxygen isotope effect in cuprate superconductors based on a phonon-mediated d-wave pairing model within the Bardeen-Cooper-Schrieffer theory. We show that this model accounts for the magnitude of the isotope exponent as functions of the doping level as well as the variation between different cuprate superconductors. The isotope effect on the superconducting transition is also found to resemble the effect of pressure on the transition. These results indicate that the role of phonons should not be overlooked for explaining the superconductivity in cuprates.electron-phonon interaction ͉ high-temperature superconductivity ͉ pressure effect U nderstanding the high-temperature superconductivity in cuprate superconductors is at the heart of current research in solid-state physics. The isotope effect is an important experimental probe in revealing the underlying pairing mechanism of superconductivity. Early measurements (1) of the isotope effect on the superconducting transition temperature, T c , provided key experimental evidence for phonon-mediated pairing and supported strongly the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity in conventional materials. When hightemperature superconductivity was discovered in copper oxides, the oxygen isotope exponents, defined by ␣ ' ϪdlnT c /dlnM, with M being the isotopic mass, were promptly measured. The initial finding of the small value of ␣ in La 1.85 Sr 0.15 CuO 4 (2, 3) and near zero value in YBa 2 Cu 3 O 7Ϫ␦ (␦ ϳ 0) (4-6) was taken to be convincing evidence against electron-phonon interaction as the dominant mechanism in these materials. However, later experiments have revealed a much richer and more complex situation (7). There is a striking variation of ␣ with doping level in a given cuprate material (8-11). Meanwhile, ␣ also varies among various compounds of different cuprate families (7,12). Interestingly, ␣ is a remarkably monotonic function of the number of CuO 2 layers in a way opposite to T c in optimally doped compounds (12). Such elaborate isotope effects strongly suggest that one should include phonons in the theory of high-T c superconductivity. Studies of neutron scattering (13), angleresolved photoemission spectroscopy (14, 15), and scanning transmission electron microscopy (16) provide further evidence for phonons being an important player in the basic physics of high-T c superconductivity.Compared with isotope substitution, pressure has been realized not only to be another effective way to change T c for a superconductor (17) but also to yield information on the interaction causing the superconductivity. So far, the record high T c of 164 K was achieved under high pressure in HgBa 2 C...

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Section: [3]supporting

confidence: 79%

“…Our high pressure Raman system and experiments are detailed in ref. 38. We have used synthetic ultrapure anvils to reduce diamond fluorescence.…”

Section: Methodsmentioning

confidence: 99%

Unified picture of the oxygen isotope effect in cuprate superconductors

Chen

Struzhkin

et al. 2007

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

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“…Noncollinear calculations show that at least for high volumes the ground state is noncollinear, but stabilization energies are small (33). One may consequently imagine a magnetic-phase transition sequence from a noncollinear ground state to the collinear afmII structure to a paramagnet with increasing temperature, which could also explain the strong decrease in amplitude and eventual disappearance of the satellite peak in experiments when the temperature is lowered (34). The absence of splitting in another set of experiments (35) can be explained by the use of natural diamonds for the cell anvils, where fluorescence significantly impairs the quality of the spectra, to the extent that the satellite peak would not be observable.…”

mentioning

confidence: 99%

Magnetism in dense hexagonal iron

Steinle‐Neumann

Stixrude

Cohen

2003

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

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The magnetic state of hexagonal close-packed iron has been the subject of debate for more than three decades. Although Mö ssbauer measurements find no evidence of the hyperfine splitting that can signal the presence of magnetic moments, density functional theory predicts an antiferromagnetic (afm) ground state. This discrepancy between theory and experiment is now particularly important because of recent experimental findings of anomalous splitting in the Raman spectra and the presence of superconductivity in hexagonal close-packed iron, which may be caused by magnetic correlations. Here, we report results from first principles calculations on the previously predicted theoretical collinear afm ground state that strongly support the presence of afm correlations in hexagonal close-packed iron. We show that anomalous splitting of the Raman mode can be explained by spinphonon interactions. Moreover, we find that the calculated hyperfine field is very weak and would lead to hyperfine splitting below the resolution of Mö ssbauer experiments. P hysical properties of iron are of great importance to many fields in the sciences, as iron is one of the most abundant and stable elements in the universe and the very basis for the steel industry. Hexagonal close-packed (hcp) iron, the form stable at high pressure, plays a central role in geophysics, as the Earth's inner core is thought to be primarily composed of this phase (1, 2), and in our understanding of impact and explosive phenomena in iron and steel (3). The magnetic state of iron has a major influence on the physics of iron and iron alloys, including the relative stability of the iron polymorphs (4, 5). The magnetic structure of the hcp phase has been the subject of a scientific debate for three decades (6), leading to contradictory results from experiments and theory. Although experiments are interpreted to show the absence of magnetism in hcp iron (6-10), computations based on density functional theory find an antiferromagnetic (afm) ground state stable to Ϸ50 GPa (11), similar to magnetism in the double hcp phase (5). The possible presence of magnetism in hcp iron as further substantiated in this article has important implications. In geophysics many experiments are carried out on potentially magnetic hcp iron and are then extrapolated to pressures of Earth's core, into the nonmagnetic region of the hcp stability field at high pressure, and may consequently not be valid. The possible presence of magnetism in hcp iron also plays an important role in the discussion of the recently observed superconductivity of hcp iron (12, 13): Magnetic correlations in hcp iron appear to be necessary to explain the observed pressure dependence of its superconductivity (14-16).The two lower-pressure polymorphs of iron are both magnetic. The phase stable around ambient conditions, body-centered cubic (bcc), owes its stability entirely to the presence of ferromagnetism (4). Heating above the Curie temperature causes the spins to disorder and the net magnetization to vanish, but the ind...

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“…Raman scattering is a very convenient technique for application with diamond anvil cell (DAC) pressure devices [18]. Indeed, few experiments have been reported at high pressure, probing antiferromagnetic interaction in NiO [19], K 2 NiF 4 [20], and also in parent high-T c compounds by measuring two-magnon Raman scattering.…”

Section: Pressure Effect On Phonon and Magnon Excitations And Pressurmentioning

confidence: 99%

Magnon–phonon coupling and implications for charge-density wave states and superconductivity in cuprates

Struzhkin

Chen

2016

Low Temperature Physics

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The mechanism of high-temperature superconductivity of copper oxides (cuprates) remains unsolved puzzle in condensed matter physics. The cuprates represent extremely complicated system, showing fascinating variety of quantum phenomena and rich phase diagram as a function of doping. In the suggested "superconducting glue" mechanisms, phonon and spin excitations are invoked most frequently, and it appears that only spin excitations cover the energy scale required to justify very high transition temperature T c ~ 165 K (as in mercury-based triple layer cuprates compressed to 30 GPa). It appears that pressure is quite important variable helping to boost the T c record by almost 30 degrees. Pressure may be also considered as a clean tuning parameter, helping to understand the underlying balance of various energy scales and ordered states in cuprates. In this paper, a review of mostly our work on cuprates under pressure will be given, with the emphasis on the interactions between phonon and spin excitations. It appears that there is a strong coupling between superexchange interaction and stretching in-plane oxygen vibrations, which may give rise to a variety of complex phenomena, including the charge-density wave state intertwined with superconductivity and attracting a lot of interest recently. PACS

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Raman spectroscopy of metals, high‐temperature superconductors and related materials under high pressure

Cited by 94 publications

References 137 publications

Unified picture of the oxygen isotope effect in cuprate superconductors

Unified picture of the oxygen isotope effect in cuprate superconductors

Magnetism in dense hexagonal iron

Magnon–phonon coupling and implications for charge-density wave states and superconductivity in cuprates

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