Pressure dependance of optical excitations in tetragonal Sr2VO4

Optical Infrared High-Pressure and Low Temperature Setup
Michaël Trana, Julien Levalloisa, Ana Akrapa, Riccardo Tediosia, Mehdi Brandta, Philippe Lerchb and Dirk van der Marela
a
Département de Physique de la Matière Condensée, Université de Genève, 24 Quai Ernest-Ansermet, CH 1211 Genève 4, Switzerland,
b
Infrared Beamline, Swiss Light Source, Paul Sherrer Institut, CH 5232 Villigen-PSI, Switzerland
Temperature-dependent optical infrared spectroscopy allows to probe low energy vibrational, electronic and magnetic excitations present in a large variety of systems. Applying pressure,
provided hydrostatic conditions are obtained, modifies the inter-atomic distances (if not the crystal structure of the material). Therefore, pressure can be thought as a «dial knob» of the
electronic band structure of materials. Thus, interactions between charge carriers can be probed, without the often-unknown effects present when chemical doping is used instead.
The setup mounted
on the 66 v/s FTIR
spectrometer at the
F3A port of the X01DC
infrared beamline of
SLS @ PSI
Example: spectra of kerosene under high pressure and at low temperature. A shift of the fitted peak positions (R.T. only) towards higher frequencies is observed.
1.4
120
1.2
Peak # 4
Peak # 3
100
80
0.8
1.6
1.4
0.6
3 kbar
52 kbar
100 kbar
fit 3 kbar
fit 52 kbar
fit 100 kbar
0.4
0.2
Peak # 1
1.2
1
60
40
100 kbar @ R.T.
105 kbar @ 185 K
103 kbar @ 100 K
98 kbar @ 22 K
0.8
0.6
20
0.4
0
Peak # 2
-0.2
Shift (cm-1)
Transmission (A.U.)
1
Peak # 1
Peak # 2
Peak # 3
Peak # 4
Fit peak # 1
Fit peak # 2
Fit peak # 3
Fit peak # 4
1000
1000
2000
3000
2000
4000
Wavenumber (cm-1)
3000
4000
5000
5000
6000
6000
0
20
40
60
Pressure (kbar)
80
100
Setup and optical path
Main components
The setup is essentially an infrared microscope working entirely in vacuum. It’s based on three essentials
parts:
• A BETSA Diamond anvil pressure cell
• A Janis ST-100 helium-flow cryostat
• A Bruker 66V Fourier-Transform Infrared Spectrometer coupled to the IR synchrotron source.
Samples
The pressure in the diamond anvil cell is tuned in situ by the use a helium-inflated membrane connected to a capillary network. The optical path can be changed by a motorized mirror/beamsplitter allowing optical access to the sample through quarzt windows. The pressure is determined by the shift
in wavelenght δλ of the R1 peak in the ruby’s fluorescence spectrum.
• Liquids
• (Very) small crystal samples or powders + a transmitting medium are loaded into a hole drilled in a
CuBe gasket along with ruby chip(s) as a pressure gauge.
!
Experimental constrains
Measurement in transmission mode
Targeted pressure range defines sample’s size
Limited observable range in the far infrared (d>5 λ)
Reduced optical throughput is compensated by the use of a synchrotron light source
When used, possible low to mid infrared
absorption the pressure-transmitting medium
Sample cannot be thicker than the gasket (~50µm)
Strong absorption in the 2000 - 2500 cm-1 range due to the two diamonds type IIa of the DAC
d ~ 250 µm
a
b
c
•
•
•
d
!
a) Schematic cut view of the entire MDAC then b & c) only of the
pair of diamond sealing the gasket’s hole. d) DAC under a regular table optical microscope: picture of the gasket’s hole loaded
with ruby chips, sample and kerosene as a pressure medium. e)
DAC inside the setup and optical access thru the right reflective
objective: gasket loaded with ruby chips and kerosene. The red
«spot» on the left is synchrotron light.
e
Spectral range: from far infrared (depending on sample’s size)
to visible & UV (with differents detectors)
Temperature: from room temperature to 22 K
Pressure: from a few kbar to 200 kbar
Outlook: performing reflectivity measurements
Reflectivity? Optic plate for T/R
Schematic 3/4 cut view of the IFS 66 v/s sample chamber with a switching optical path for transmission and reflectivity measurements
Schematic 3/4 cut view of the whole setup.
Coldfinger
MDAC
15x Reflective
Objective
Mirror/BS/Clear
Laser
Fluoresence of ruby under pressure
3500
R1
2500
2500
am
2000
su
re
i
nc
rea
1000
se
500
6930
500
6800
Pre
s
2000
1500
1000
senc
e
I R Be
R2
1500
Fluo
re
3500
3000
3000
A.U.
1.
2.
3.
4.
5.
6.
7.
,ESô0ÙLESôDEôRECHERCHEôNAT
GÏNÒRENTôUNEôRECHERCHEôDE
ETôCRÏEôDEôLAôVALEURôAJOUTÏE
HISTOIREôESTôCELLEôDUNôGRAN
HELVÏTIQUE
Within the spectrometer’s sample chamber a plane mirror deflects the infrared beam through KBr
sealing windows to an upper chamber, were another plane mirror steers light into a 15X magnification
reflective Cassegrain objective. The sample mounted into the DAC which is itself attached to the cold
finger of the Janis cryostat, can be adjusted into position by an XYZ stage. Transmitted light is collected by a another Cassegrain objective and send to the detector along a symmetrical optical arrangement. Sample temperature is measured at the end of the coldfinger as well as within the pressure cell.
Temperature regulation is achieved by adjusting the helium flow together with the electrical power
dissipated in a cartridge heater attached to the cold finger.
6900
6935
7000
1904
P (δλ, R1 , B) =
B
6940
λ [Å]
6945
6950
7100
δλ
1+
R1
6955
6960
6965
7200
B
−1
PAUL SCHERRER INSTITUT