Daniel S. Dessau

Professor, Ph.D. Stanford University, 1993.
Dessau@Colorado.edu
Duane Physics room F625
303-492-1607 (office)       303-492-3308 (lab)       303-492-2998 (fax)

I am a spectroscopist using a variety of tools for the study of the electronic structure, magnetic structure, and phase transitions of novel materials systems such as high temperature superconductors (HTSCs or cuprates); topological insulators, iridates, and strong spin-orbit materials; and colossal magnetoresistive oxides (CMRs or manganites).  We are users of  synchrotron light facilities around the world and also have unique ARPES (angle resolved photoemission spectroscopy) and Laser-ARPES instrumentation in our home lab.

Orbital texture switching and first demonstration of coupled spin-orbital texture in Toplogical Insulators

Topological insulators such as Bi2Se3 are novel materials with "protected" spin-polarized metallic surface states around an insulating bulk interior.  These surface states have an unusual "Dirac cone" structure that meets at a point (the Dirac point).  Previously it had been believed that all the interesting physics of these Dirac surface states was in the spin part of the wavefunction, but we recently showed that the orbital part is also very interesting. We perfomed ARPES at the Advanced Light Source, Berkeley using s-polarized photons which highlights the in-plane orbitals of the Dirac cone, which were almost universally ignored previously.  The equal-energy surfaces are broken up into two semicircles, which are arranged left-right for the states above the Dirac point (DP) and up-down for the states below the DP. Our results highlighted the importance of the (typically ignored) in-plane orbitals (panel c, below), with the intensity maps of panel a indicating that these in-plane orbitals have an alignment that is tangential to the constant energy surfaces above the DP, transitioning to radial to the surfaces below the DP (panel c). Working together with the density functional theory group of Alex Zunger we showed that the orbital texture switch occurs exactly at the DP.  See“Mapping the orbital wavefunction of the surface states in three-dimensional topological insulators”, Nature Physics 9, 499–504 (2013).
Bi2Se3 orbital symmetry
Orbital selective ARPES data of the topological insulator Bi2Se3, showing constant energy cuts at a variety of energies relative to
the Dirac point (energies below the panels) with s and p polarization.   The data indicates that the in-plane orbitals have a an orbital symmetry switch
from tangential above the DP to radial below it (panel c). From Nature Physics 9, 499 (2013)

Following our observation of the various orbital wavefunctions above, Shoucheng Zhang of Stanford realized that this would imply that the in-plane orbital parts of the wavefunction could have a different spin texture than the out-of plane orbital parts, and that this could be measured with spin-ARPES. These results showed for example the in-plane orbitals above the DP have a right-handed or “backwards” spin texture (panels c,e,g, below) compared to the dominant left-handed texture, which is observed for the pz orbitals (panels b,d,f).  Taken together, these results show that it is the total angular momentum Jz that is conserved, rather than the spin polarization as had been previously assumed – something that makes sense in a system with strong spin-orbit coupling. See “Coupled Spin-Orbital Texture in a Prototypical Topological Insulator” arXiv:1211.5998.
Bi2Se3 Spin Orbital Texture
A combination of spin polarized and orbital-selective ARPES of the topological insulator Bi2Se3.
 The data shows that the in-plane orbitals have a "backwards" right handed spin polarization above the DP, indicating that it is the angular momentum Jz rather than the spin that is conserved.
From arXiv:1211.5998.

Pairing and pair-breaking scattering in cuprate high temperature superconductors, and our new TDoS technique.

ARPES has been used to measure the superconducting gaps (pairing energies) of cuprate superconductors for more than 20 years, with this arguably the most powerful and direct of spectroscopies for this.  Despite this, gap measurements still used highly approximate methods because we do not yet have an understanding of the ARPES lineshape in the absence of the gap so we did not have the correct “reference” spectrum that is altered by the superconductivity. To combat this lineshape uncertainty we have developed a new method of ARPES analysis that effectively integrates out the unknowns of the lineshape, leaving us with simple structures that are much better understood and can be analyzed in a straightforward way.  We call this new technique the Tomographic Density of States or "TDoS" technique because it deals with slices (hence tomography) of data in momentum space.  Our first paper on this showed how to create the TDoS spectrum and how to accurately extract the pair-breaking scattering rate Gamma as well as the pairing energy ∆.  We also showed how the interplay between these two rates helps explain the very unusual “Fermi arcs” that had long been discussed in the literature. We believe that this is presently the most direct and accurate method of all spectroscopies to measure gaps in the cuprates, and in addition it also gives us quantitative information about the pair-breaking scattering rates, which were not available from ARPES before, and not available in a k-resolved way from any other spectroscopy.  See  T. J. Reber et al, “The Non-Quasiparticle Nature of Fermi Arcs in Cuprate High-TC Superconductors” Nature Physics 8, 606–610 (2012)

Ferni Arcs in BSCCO
                            observed from the TDoS ARPES technique


First Demonstration of  true Laser-ARPES

We have recently made the first demonstration of true Laser-ARPES (Angle Resolved Photoemission Spectroscopy) at our home lab in Colorado.  The data below shows the measured dispersion (Energy vs. momentum) for electronic states from the high transition temperature superconductor Bi2Sr2CaCu2O8 along the "nodal" direction of the Brillouin zone.  The two right panels were taken at the synchrotron (Advanced Light Source, Berkeley) using their most powerful undulator beamlines.  The left panel was taken using our laser-ARPES system and shows dramatically sharper spectral peaks.  This increase in the spectral resolution is due to improvements in the energy and momentum resolution of the experiment, but also may be due to other issues such as a decrease in the final-state electron broadening effects and an improved bulk sensitivity.  The increase in the spectral resolution can give a much greater window into the inherent electronic physics of these exotic materials, allowing us to measure the details of the "quantum dance" that the electrons are undergoing in these materials.  Ultimately we hope this kind of information will enable us to uncover the interactions responsible for the superconductivity in these amazing materials.  Click here for a reprint of this work published in Physical Review Letters recently.  A news article highlighting this data also recently appeared in Science.

ARPES data from the "nodal" line of the high temperature superconductor Bi2Sr2CaCu2O8. (Left) laser-ARPES data compared to data we measured at the Advanced Light Source (middle and right). From Science 310, 1271 (2005)

 

AntiNodal "kinks" in Cuprate Superconductors

We used high resolution ARPES to make the first clear observation of dispersion "kinks" in the antinodal region (where the superconducting pairing is strongest) of a high Tc superconductor.  This kink is only present in the superconducting state and indicates significantly stronger coupling and a lower energy scale than the "nodal kink" previously observed in the cuprates, where the superconducting pairing is absent.  We argue that the origin of this effect is likely the magnetic resonance mode observed by inelastic neutron scattering, greatly increasing the possibility that this mode helps mediate the pairing.  If so, this would be in opposition to the case of conventional superconductivity, where the superconducting pairing is mediated by phonons (quantized sound waves).  Shown below is a figure from our article in Nature.  An other paper describing this in more detail is available here.

 

 

Fermi Surface and Pseudogaps in CMR Oxides

 

We used high resolution ARPES to make the first measurements of the Fermi Surface of a CMR oxide (panel A, below).  Even though these measurements show a clear Fermi Surface the electronic spectral weight at the Fermi level is very small, which is surprising since these measurements were made in the metallic state.  This very small weight at EF is termed a pseudogap, and has become one of the central topics of modern condensed matter research.  Shown below is a figure from our paper  published in Science.