Hidden magnetic excitation in the pseudogap phase of a high-Tc superconductor

• Published in: Nature (London) Vol. 468; no. 7321; pp. 283 - 285
• Main Authors: Li, Yuan, Balédent, V., Yu, G., Barišić, N., Hradil, K., Mole, R. A., Sidis, Y., Steffens, P., Zhao, X., Bourges, P., Greven, M.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 11-11-2010; Nature Publishing Group
• Subjects: 639/766/119; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Humanities and Social Sciences; letter; multidisciplinary; Phase diagrams; Physics; Properties of type I and type II superconductors; Science; Science (multidisciplinary); Superconductivity; Energy gap; Phase diagrams; Spin waves; Quantum phase transition; Inelastic scattering; Brillouin zones; High-Tc superconductors; Collective mode; Barium Copper Mercury Oxides Mixed; Neutron diffusion
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature09477

The pseudogap enters a new phase
The pseudogap phenomenon, a discontinuity in the energy level of a material's electronic spectrum, is a universal characteristic of the high transition temperature (
T
c
) copper oxides. The nature of the pseudogap has been a central question in condensed matter physics for more than a decade, but many of its properties remain unexplained. Recent studies have pointed to the universal existence of an unusual magnetic order below
T
*, the temperature below which the anomalous properties associated with the pseudogap become apparent. If confirmed, this would have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. The results of inelastic neutron scattering experiments on the superconductor HgBa
2
CuO
4+δ
(Hg1201) now reveal a fundamental collective magnetic mode associated with the unusual order, providing further support for this picture.
Recent findings indicate that the pseudogap regime in the high-transition-temperature copper oxides constitutes a new phase of matter rather than a mere crossover phenomenon. These authors report inelastic neutron scattering results for HgBa
2
CuO
4+δ
that reveal a fundamental collective magnetic mode associated with the unusual order, further supporting this picture. The mode's intensity rises below the pseudogap characteristic temperature and its dispersion is weak.
The elucidation of the pseudogap phenomenon of the high-transition-temperature (high-
T
c
) copper oxides—a set of anomalous physical properties below the characteristic temperature
T
* and above
T
c
—has been a major challenge in condensed matter physics for the past two decades
1
. Following initial indications of broken time-reversal symmetry in photoemission experiments
2
, recent polarized neutron diffraction work demonstrated the universal existence of an unusual magnetic order below
T
* (refs
3
,
4
). These findings have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. They are furthermore consistent with a particular type of order involving circulating orbital currents, and with the notion that the phase diagram is controlled by a quantum critical point
5
. Here we report inelastic neutron scattering results for HgBa
2
CuO
4+
δ
that reveal a fundamental collective magnetic mode associated with the unusual order, and which further support this picture. The mode’s intensity rises below the same temperature
T
* and its dispersion is weak, as expected for an Ising-like order parameter
6
. Its energy of 52–56 meV renders it a new candidate for the hitherto unexplained ubiquitous electron–boson coupling features observed in spectroscopic studies
7
,
8
,
9
,
10
.