High-temperature (T_c) superconductivity is one of the most important topics in condensed matter physics. Despite extensive studies over more than two decades, the microscopic mechanism of high temperature superconductivity still remains elusive due to many unconventional properties that are not well understood. Among them, the most mysterious behavior of high-T_c superconductor is the nature of so called "pseudogap", which has been a focus of the field for many years. In conventional superconductors, a gap exists in the energy absorption spectrum only below T_c, corresponding to the energy price to pay for breaking a Cooper pair of electrons. In high-T_c cuprate superconductors, an energy gap called the pseudogap exists above T_c but below T^*, and is controversially attributed either to pre-formed superconducting pairs or to competing phases. Recently, by carefully studying the "symmetry" of the gap, researchers Makoto Hashimoto and Rui-Hua He, along with their co-workers in Prof. Zhi-Xun Shen's group at Stanford University, have found crucial evidence suggesting that the particle-hole symmetry required by superconductivity is broken in the pseudogap state.

The measurements were performed at SSRL Beam Line 5-4 using the state-of-the-art angle-resolved photoemission spectroscopy (ARPES) system with excellent beam and endstation stability. With this incomparable experimental setup, Hashimoto and He et al successfully obtained a high quality data set of the detailed temperature dependence of the pseudogap in a high-temperature cuprate superconductor Bi2201. By covering a wide temperature range from below T_c (34 K) to above T^* (125 K) in the antinodal region close to the Brillouin zone boundary where the pseudogap reaches its maximum value, many important insights on the nature of the pseudogap were revealed which have not been achieved by previous ARPES studies.

In this work published in Nature Physics, the "symmetry" of the pseudogap was explored by examining the dispersion of the occupied electronic states measured by ARPES. As shown in Fig. 1m, when a particle-hole symmetric gap opens from the normal state dispersion (red curve) due to superconductivity, one always expects an alignment between the Fermi momentum k_F and the "back-bending" (green arrows) of the dispersion in the gapped states (weighted blue curve). Because of this strong constraint, the observation of back-bending away from k_F in a gapped state can be taken as conclusive evidence for a broken particle-hole symmetry nature of the gap, even though the information on the unoccupied state is absent in ARPES spectra. Therefore, a close examination of the symmetry of the gap may help us to identify the origin of the pseudogap: whether is due to pre-formed superconducting pairs or competing phases.

Figure 1. Particle-hole symmetry breaking in the antinodal dispersion of pseudogapped Pb-Bi2201. Tc = 34 K, T* = 125 ± 10 K. a-l, Fermi-Dirac function (FD) divided image plots (upper panels) and corresponding spectra as a function of parallel momentum (lower panels) taken along the antinodal cut shown in the inset of g at selected temperatures. The intensity maximum of each spectrum is marked by circle. Spectra in red and green are at kF and back-bending momenta of the dispersion, respectively. m, Simulated dispersion for d-wave homogeneous superconductivity with order parameter V = 30 meV. Cuts are along (pi, - pi)-(pi, 0)-(pi, pi). The red (blue) curve is for the true normal (gapped) state. Spectral weight is indicated by the curve thickness. The back-bending (or saturation) of the dispersion and kF are indicated in the panel. Note that the back-bending momentum in the gapped state remains aligned with kF. n, Summary of the intensity maximum dispersions at different temperatures.