Complex evolution of the electronic structure of Cr with temperature
Ganesh Adhikary, R. Bindu, Swapnil Patil, and Kalobaran Maiti
a)
Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research,
Homi Bhabha Road, Colaba, Mumbai - 400 005, India
(Received 1 November 2011; accepted 23 December 2011; published online 23 January 2012)
Employing state-of-the-art high resolution photoemission spectroscopy, we studied the electronic
structure evolution of Cr with temperature. Experimental results reveal signature of a pseudogap
much below the spin density wave transition temperature. A sharp peak appears near the Fermi
level at low temperatures presumably related to the orbital Kondo effect. These results provide
possible origin of the complex electronic properties observed in this system.
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2012 American
Institute of Physics. [doi:10.1063/1.3678183]
Among elemental metals, chromium is one of the most
important element, which is used in varieties of applications
such as metallurgical applications, making dye and pig-
ments, catalysis, tanning, refractory materials, magnetic
tapes and recording media, thermocouple etc., due to its
interesting but complex electronic properties. For example,
Cr is an archetypical example of Fermi surface nesting
driven antiferromagnet
1
exhibiting incommensurate spin
density wave (SDW) transition at 311 K, which can be sup-
pressed to 0 K by small ($3.5%) V doping.
2
Various studies
derived the Fermi surface nesting vector in the SDW phase
for both bulk and surface electronic structures.
3–6
An infra-
red reflectivity study predicted multiple gaps (a direct gap of
125 meV and an indirect gap of 450 meV (Ref. 7)) or pseudo-
gap in SDW phase as predicted theoretically.
8
A scanning
tunneling microscopic measurements showed orbital Kondo
resonance in Cr.
9
Subsequent studies
10,11
indicated varied
scenarios involving Kondo interactions, electron-phonon
coupling, Shockley-type surface state formation, etc. Evi-
dently, an elemental metal, Cr possess complexity associated
to widely discussed quantum phase transitions in unconven-
tional superconductors, heavy fermions etc.
Here, we studied temperature evolution of the electronic
structure of Cr employing state of the art high resolution
photoemission spectroscopy. Measurements were carried out
on high purity in situ scraped sample using monochromatic
He
I
(21.2 eV), He
II
(40.8 eV), and Al Ka (1486.6 eV) pho-
ton sources and SES2002 Gammadata Scienta analyzer with
the energy resolution set to 2.5 meV, 5 meV, and 350 meV,
respectively, at a base pressure <3 Â 10
À11
Torr. The energy
dispersive analysis of x-rays using a scanning electron
microscope (SEM) and photoemission measurements do not
exhibit detectable impurity suggesting the level of impurity,
if any, is below 1%. SEM images of as prepared surface
shown in Figs. 1(a) and 1(b) exhibit a smooth surface. The
SEM image of the sputtered—annealed surface is shown in
Fig. 1(c) exhibiting improved surface quality. However, the
bulk photoemission spectra were found not to be influenced
by the surface preparation as expected for such measure-
ments with a spot size of about 1 mm.
In Fig. 1(d), we show the energy band structure of Cr
calculated using full potential linearized augmented plane
wave method within the local spin density approximations.
12
4s and 4p contributions appear at higher binding energies
(>3 eV) and the Fermi surface is formed by 3d electronic
states. Around C point, the energy bands forming electron
pockets has t
2g
symmetry and hole pockets possess e
g
sym-
metry. The SDW gap, observed earlier,
4
forms via folding of
the t
2g
bands while the e
g
bands cross the Fermi level,
F
retaining the metallicity at low temperatures. We have con-
voluted the sum of the photoemission cross section weighted
partial density of states with the Fermi-Dirac distribution
function to extract the occupied part and a gaussian repre-
senting the energy resolution for Al Ka measurements.
Resulting spectrum shown in Fig. 1(e) captures almost all
the features of the Al Ka valence band spectrum—a weak in-
tensity around 6 eV shown by the arrow in the figure may
appear due to asymmetric lineshape arising from energy de-
pendent life time broadening, low energy excitations in the
photoemission final states, or a signature of electron correla-
tion induced lower Hubbard band
13
as found in other transi-
tion metals such as Ni.
14
Correlation effect cannot be
ignored as it possess finite magnetic moment—a result of
effective local character of 3d electrons. The signature of
electron correlation also appears as satellites in the core level
spectra.
15
The Cr 2p spectrum shown in Fig. 1(f) exhibits
two features corresponding to the spin-orbit split 2p
3/2
and
2p
1/2
signals with intensity ratio commensurate to their mul-
tiplicity of 2:1. No intensity for the correlation induced satel-
lite is observed suggesting that the electron correlation, if
present in this system, is significantly weak.
16
Since the valence electronic states possess essentially Cr
3d character, the photoemission cross-section will be a multi-
plicative factor to intensity. Therefore, a comparison of the
valence band spectra collected with different photon energies
(see Fig. 2(a)) manifests the surface-bulk differences in the
electronic structure.
17
To consider the differences in resolu-
tion broadening in various experimental techniques, we
superimposed the resolution broadened He
I
spectrum (line)
over the Al Ka spectrum (open circles). The He
I
spectrum
exhibits higher intensities of the features S1 and S2, while
features B1 and B2 are dominant in the Al Ka spectrum. The
electron mean free path at ultraviolet energies is lower than
that in x-ray energies.
17
Thus, S1 and S2 can be identified as
a)
Author to whom correspondence should be addressed. Electronic mail:
kbmaiti@tifr.res.in.
0003-6951/2012/100(4)/042401/4/$30.00
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2012 American Institute of Physics100, 042401-1
APPLIED PHYSICS LETTERS 100, 042401 (2012)