You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
almost all properties of a material are given by its electronic structure .
for metallic materials , the most important characteristic of the electronic structure is the fermi surface , namely the boundary between occupied and unoccupied states in momentum space .
the traditional experimental method for studying the fermi surface is the de haas - van alphen effect , which is limited to low temperatures , and therefore can not be utilized for the study of certain paramagnetic materials , for example cr , since the nel temperature of cr is @xmath5k @xcite .
for this reason , numerous acar investigations , both experimental @xcite and theoretical @xcite have been performed on paramagnetic cr .
studying the fermiology of the paramagnetic phase of cr would allow to gain insight to the driving mechanism of the anti - ferromagnetic ordering of cr below @xmath5k , since a nesting feature of the fermi surface gives a possible explanation .
however , several 2d - acar investigations performed in the paramagnetic @xcite phase of cr revealed significant discrepancies between the measured and the calculated two - photon - momentum distribution , which can not be explained in terms of an unexpected topology of the fermi surface . these discrepancies are most probably due to @xmath6-@xmath7-correlations and/or many - body effects not accounted for by theory .
recently , an experimental approach to this problem , the utilisation of a state - dependent enhancement has been presented in@xcite . with this method the enhancement is directly obtained by fitting the contribution of the individual bands to the data .
this way , the fermi surface nesting vector was deduced via the computation of the static susceptibility from the fitted lmto calculations . in our study
we focussed on the differences in the fermi surface topology between paramagnetic and anti - ferromagnetic cr .
therefore , 2d - acar measurements at different temperatures have been performed .
the 2d - acar spectrometer consists of two main components , the source - sample chamber including the sample environment , and the detector assembly consisting of two anger - type gamma cameras , which were obtained from the positron group of bristol university .
the anger cameras are positioned symmetrically at a distance of @xmath8 m to the source - sample chamber .
the munich 2d - acar spectrometer features two interchangeable sample holders , with an available temperature range between @xmath3k and @xmath2k , both of which were used for this study .
the details of the munich 2d - acar spectrometer and the performance characteristics can be found in @xcite .
+ for this study , four 2d - acar measurements have been conducted , each with the integration direction chosen along [ 100 ] .
two measurements were performed with the coolable sample holder in the anti - ferromagnetic phase of cr , one at @xmath3k with a total of @xmath9 counts and the other one at room temperature with a total of @xmath10 counts .
two additional measurements were performed with the heatable sample holder , one at room temperature , in order to ensure reproducibility , with a total of @xmath11 counts and the other one at @xmath4k with a total of @xmath12 counts . before each of the two measurement campaigns the orientation of the cr crystal was checked by means of laue diffractometry and aligned to @xmath13 . before the sample was mounted the surface was etched with concentrated hydrochloric acid in order to remove the oxide layer from the surface .
the crystal was cut by means of spark erosion , therefore it is assumed that the crystal is defect free .
positron lifetime measurements are underway to investigate the defect concentration . + the recorded acar spectra were corrected for the momentum sampling function and then folded back to * k*-space following the lcw - procedure@xcite .
this way features in the projection of the momentum distribution attributed to the fermi surface are enhanced and easier to identify .
the lcw - folded data were corrected for background contributions and scaled from @xmath14@xmath15 so that the dynamic range is identical for all three measurements as well as for theoretical calculations : @xmath14 represents unoccupied states while @xmath16 represents the occupied states .
+ in addition , first principle band structure calculations have been performed for paramagnetic cr using the munich spr - kkr - package @xcite and the expected two - photon momentum distribution was deduced . for the calculations
the independent particle model was applied , therefore @xmath6@xmath7correlation effects are not included in the theoretical data .
lcw distribution for cr [ 100 ] at @xmath4k . slightly above the nel temperature for cr of @xmath5k . ]
lcw distribution for cr [ 100 ] at @xmath4k . slightly above the nel temperature for cr of @xmath5k . ]    cut through the lcw - folded projections along @xmath17-n-@xmath17 ( p@xmath18p@xmath19 ) for the paramagnetic and the anti - ferromagnetic phase .
the initial lcw - projections were normalized to the same amount of counts . ]    cut through the lcw - folded projections along @xmath17-n-@xmath17 ( p@xmath18p@xmath19 ) for the paramagnetic and the anti - ferromagnetic phase .
the initial lcw - projections were normalized to the same amount of counts . ]
two of the four measurements were performed at room temperature in order to check if the acar spectra are consistent before and after the change of the sample holder . since both spectra are compatible within the statistical limits only the spectrum with the higher statistics
is presented here ( see figure [ fig : room ] ) .
the comparison of the low temperature measurement and the room temperature measurement shows the effect of decreasing resolution with increasing temperature .
however , besides the lower resolution in the room temperature measurement both spectra are comparable , therefore only one is shown .
+ the features at the n - points appear to be occupied in the anti - ferromagnetic phase ( high symmetry points of the brillouin zones of both phases , _ simple cubic _ for the anti - ferromagnetic phase and _ bcc _ for the paramagnetic phase are indicated in figures 15 ) .
this picture changes when the sample temperature is increased above the nel temperature ( see figure [ fig:318 ] ) .
a small difference in temperature drives the sample into the paramagnetic state and yields a drastic change in the lcw - folded data .
the n - hole pockets become more pronounced , which can be seen comparing cuts along @xmath17-n-@xmath17 for both phases ( see figure [ fig : cut1 ] ) . furthermore
, the electron pocket structure at the x - points connecting the n - points is smaller than in the anti - ferromagnetic phase .
generally , the agreement of our measurement with previous 2d - acar measurements ( see ref.@xcite ) on paramagnetic cr is considerable .
however , the discrepancies between our data and the sprkkr calculations are quite substantial .
this has also been reported from different laboratories @xcite .
the relative intensities in the calculation for the occupied and the unoccupied states can not be reproduced in the lcw - folded data ( see figure [ fig : rech ] ) .
the experimental resolution of our spectrometer lies between @xmath20mrad and @xmath21mrad @xcite , which is only slightly less than the extent of the n - point features in our data .
however , the changes in the lcw - folded data when the sample is driven over the magnetic phase transition can not be attributed to resolution effects , since a temperature increase of @xmath22k results in a degradation of the overall resolution of less than @xmath23mrad .
+ during data analysis the lcw - folding was applied to the isotropic data , which was obtained by azimuthal averaging of the original data .
the isotropic data does not contain the full information on the fermi surface topology . with the isotropic data alone features at the x - point , similar to those described above , can be produced by the lcw - folding , as can be seen in figure [ fig : cut ] .
+ a possible explanation would be that the positron annihilation with a high lying flat band close to the fermi energy is strongly enhanced .
also it has to be considered the application of the lcw - theorem is only valid if the positron wave function can be characterized as a plane wave , which is not true if strong correlation effects are present , like it is the case for cr @xcite .
+ to summarize , we were able to reproduce the results from previous measurements@xcite we also could show , that the 2d - acar spectrum is drastically different in the anti - ferromagnetic and in the paramagnetic phase . however , the interpretation of our data with respect to the calculation remains an unsolved challenge .
cut through the lcw - folded data along @xmath24 ( p@xmath25 ) in comparison with the same cut for a lcw - folding of the isotropic data .
the position of high symmetry points is indicated . ]
for the interpretation of the 2d - acar data of cr a solid understanding of the enhancement is needed .
we plan on recording multiple 2d - acar projections of cr in the paramagnetic phase in order to perform a full 3d reconstruction of the momentum density , respective measurements are under way . this way
, we hope to get a better grasp of the enhancement in cr and with that gain insight to the electronic structure .
this project is funded by the deutsche forschungsgesellschaft ( dfg ) within the transregional collaborative research center trr 80 `` from electronic correlations to functionality '' .
9 steinitz m o 1986 _ j. magn .
magn . mater . _
* 60 * 137 - 144 shiotani n et al .
1977 _ j. phys .
jpn . _ * 43 * 4 singh a k and singru r m 1983 _ j. phys .
* 13 * 2189 - 2196 hughes r j et al .
2004 _ phys .
b _ * 69 * 174406 singh a k et al . 1988
_ europhys .
lett . _ * 6 * 67 sundamrajant v , kanheret d g and singru r m 1992 _ j. phys . : condens
. matter _ * 4 * 89754988 rubaszek a et al .
2001 _ phys.rev .
b _ * 65 * 125104 dugdale s b et al . 1998 _ j. phys . : condens . matter _ * 10 * 10367 biasini m 2000 _ physica b _ * 275 * 285 - 294 laverock j et al . 2010 _ phys.rev . b _ * 82 * 125127 ceeh h et al . 2013 _ rev .
_ * 84 * 043905 lock d g , crisp v h c , west r n 1973 _ j. phys .
phys . _ * 3 * 561 ebert h , kodderitzsch d , minr j 2011 _ rep .
phys . _ * 74 * 096501 rabou lp l m and mijnarends p e 1984 _ solid state commun . _
* 52 * 933 - 936
the munich 2d - acar spectrometer at the maier - leibnitz accelerator laboratory in garching has recently become operational . in the present implementation 
 a 2d - acar spectrometer is set up , with a baseline of @xmath0 m , a conventional @xmath1na positron source and two anger - type gamma - cameras . 
 the positrons are guided onto the sample by a magnetic field generated by a normal conducting electromagnet . 
 the sample can be either cooled by a standard closed - cycle - cryostat to low temperatures or heated by a resistive filament to temperatures up to @xmath2k . 
 we present the key features of this new 2d - acar spectrometer and , in addition , discuss first measurements on the pure metal system cr . 
 the 2d - acar measurements have been performed on cr at different temperatures : at @xmath3k and at room temperature in the anti - ferromagnetic phase and at @xmath4k slightly above the paramagnetic phase transition .