Tuning the ground state of strongly interacting low-dimensional electron systems by pressure: the cuprate spin ladder compounds
Final Report Abstract
Within this project the charge dynamics of the quasi-one-dimensional two-leg ladder compounds Sr14−x Cax Cu24 O41 for x=4 and x=11.5 has been studied as a function of temperature and external pressure by reflectivity measurements in the infrared range. The compound Sr10 Ca4 Cu24 O41 shows an insulator-to-insulator transition during cooling down related to the development of an unconventional charge-density-wave (CDW). The influence of external pressure on the CDW ground state in Sr10 Ca4 Cu24 O41 has been studied by optical reflectivity measurements as a function of temperature (10 - 300 K) and pressure P (0.3 - 4.3 GPa) over the spectral range 580 - 6000 cm−1 . With increasing pressure the CDW transition temperature TCDW decreases with the linear pressure coefficient ≈-70 K/GPa, and above ≈3 GPa the CDW phase is suppressed at all temperatures. This behavior is similar to that in compounds Sr14−x Cax Cu24 O41 with increasing Ca content x at ambient pressure, with the simple scaling x ≈ 3 · P (GPa). The size of the CDW gap decreases with increasing pressure, whereas the dimensionality of the high-temperature insulating phase in Sr10 Ca4 Cu24 O41 within the ladder plane is hardly affected by external pressure. For Ca content larger than x=9 the CDW ground state is suppressed and superconductivity (SC) is observed under high pressure. Dc resisitivity measurements as a function of temperature and pressure predict that the materials Sr14−x Cax Cu24 O41 for high Ca content become twodimensional under high pressure and at low temperature, and hence the SC state is expected to be two-dimensional in nature. Within this project the anisotropy of the charge dynamics in the material Sr2.5 Ca11.5 Cu24 O41 has been studied by polarization-dependent reflectivity measurements as a function of pressure and temperature. According to our results the material stays highly anisotropic up to at least 4 GPa independent of temperature. At room temperature the anisotropy in the charge dynamics prevails up to at least 15 GPa. The distribution of the holes among the ladder and chain subsystems as a function of Ca doping, temperature, and pressure is one important issue in Sr14−x Cax Cu24 O41 , since it determines its physical properties. To be able to follow the transfer of holes from the chains to the ladders, reflectivity measurements over a broad frequency range covering the far-infrared up to the ultraviolet are necessary. The available UV-VIS microspectrometer available by the time of the application submission had to be modified after the first test measurements. A new optical design for the UV-VIS setup was developed within the frame of this project by using the commercial ray-tracing software Zemax. This new microspectrometer was built up and test measurements were carried out.
Publications
- Suppression of the Charge-Density-Wave State in Sr10Ca4Cu24O41 by External Pressure. Phys. Rev. B.
C. A. Kuntscher, A. Huber, and M. Hücker
- Charge-density-wave state in the ladder compound Sr10Ca4Cu24O41 under high pressure, DPG Spring Conference, March 2013
A. Huber, R. Klingeler, M. Hücker, B. Büchner, and C.A. Kuntscher
