Chromium at high pressures: Weak coupling and strong fluctuations in an itinerant antiferromagnet

Abstract

The spin-and charge-density-wave order parameters of the itinerant antiferromagnet chromium are directly measured with nonresonant x-ray diffraction as the system is driven toward its quantum critical point with high pressure using a diamond anvil cell. The exponential decrease of the spin and charge diffraction intensities with pressure confirms the harmonic scaling of spin and charge, while the evolution of the incommensurate ordering vector provides important insight into the difference between pressure and chemical doping as means of driving quantum phase transitions. Measurement of the charge density wave over more than two orders of magnitude of diffraction intensity provides the clearest demonstration to date of a weakly coupled BCS-type ground state. Evidence for the coexistence of this weakly coupled ground state with high-energy excitations and pseudogap formation above the ordering temperature in chromium, the charge-ordered perovskite manganites, and the blue bronzes, among other such systems, raises fundamental questions about the distinctions between weak and strong coupling.

Hidden one-dimensional spin modulation in a three-dimensional metal

“…At 16.1 GPa, the pressure inhomogeneity across our sample is estimated to be either 0.30 GPa FWHM using a comparison of the FWHM of lattice diffraction with that of the instrument resolution (Fig. 1), accounting for the sample-specific bulk moduli 28 , or a full range of ± 0.19 GPa from previous pressure chamber studies under similar conditions 21 . The pressure inhomogeneity is typically linearly proportional to the absolute pressure in our assembly 21 .…”

Photoinduced Enhancement of the Charge Density Wave Amplitude

“…This surprisingly long time scale indicates anharmonic phonon-phonon interaction as the dominant decay channel. Electron-hole pair excitation, which typically leads to strong damping of order-parameter oscillations in strongly correlated electron systems [32,40,41], is expected to be ineffective here because of the SDW gap in the electronic spectrum [42]. Because of ultrafast carrier cooling and recondensation in the lattice distortion potential, this gap will quickly reopen, even after a complete quench of the electronic order [17].…”

“…Because of ultrafast carrier cooling and recondensation in the lattice distortion potential, this gap will quickly reopen, even after a complete quench of the electronic order [17]. Moreover, it is likely to persist above T N in the form of a pseudogap due to incipient magnetic order [42]. Finally, the period of the lattice oscillation is much shorter than the damping time and also does not depend on fluence [23].…”

Diffraction line-shapes, Fermi surface nesting, and quantum criticality in antiferromagnetic chromium at high pressure (invited)

“…1,7,8,11 In Fig. 2 we present radial scans through the pair of CDW diffraction peaks ͑2−2␦ ,0,0͒ and ͑2+2␦ ,0,0͒ together with the lattice ͑200͒ reflection.…”

“…The nesting vector Q is incommensurate with the bcc crystal lattice and varies smoothly with temperature ͑T͒, pressure ͑P͒, and light chemical doping. 7 The incommensuration is measured by a small parameter ␦ =1−Q ϳ 0.05, where both ␦ and Q = ͉Q͉ are measured in reciprocal lattice units. The incommensurate SDW gives rise to a concurrent charge density wave ͑CDW͒ which is modulated by wavevector 2Q and which couples the magnetism to the lattice.…”