2003 NSLS Users' Meeting

Spectroscopy in High Magnetic Fields: ESR, Infrared and Other Applications

Location: Bldg. 463 - Biology Seminar Room

Date: May 19, 2003

Organizer: Laszlo Mihaly, S.U.N.Y. at Stony Brook

Description: The availability of high field magnets, combined with the development of high resolution/low energy spectroscopic techniques, provides new opportunities for probing materials with the synchrotron light. In this workshop a few selected applications of X-ray and infrared radiation for the study of superconductors, magnetic perovskites, semiconductor quantum wells and other systems will be reviewed. X-ray scattering and spectroscopy, electron spin resonance, optically detected Hall effect and far IR spectroscopy in high magnetic fields will be discussed. The speakers will include current users as well as other leading experts from the US and Europe.

Schedule:

9:00 a.m. Prof. Laszlo Mihaly, SU.N.Y. at Stony Brook, Introductory Remarks

9:05 a.m. Dr. S. DeBrion, ESRF, Grenoble, France
“Orbital and Magnetic Order in Oxides, Probed by High Frequency Electron Spin Resonance”
The combination of high magnetic field and multi frequency in Electron Spin Resonance measurements is a powerful tool in solid state physics both for the determination of single ion properties ( electronic state with its caracteristic g factor anisotropy and level splitting)as well as collective magnetic excitations (magnons at q=0). We will give two examples where we have used this technique: the layered S=1/2 triangular oxides NaNiO2 and LiNiO2 and the charge ordered manganites Nd(Pr)0.5Ca0.5MnO3.

In the family of magnetic oxides special attention has been paid to LiNiO2. Its interest liesboth in its application as a positive material for lithium batteries and in its fundamentalproperties (orbital and magnetic order), which gave rise to multiple interpretations and are still the subject of constant debate. In particular the absence of orbital and magnetic order is still a mystery. The comparative study with the isomorphic compound NaNiO2 which shows orbital and magnetic order, is aimed at understanding these fundamental properties. In manganites Nd(Pr)0.5Ca0.5MnO3, charge ordering of Mn3+ and M4+ ions occurs below 250K. However a disordered, metallic state may prevail for instance when a magnetic field is applied or when the balance between Mn3+ and Mn4+ is modified. It gives rises then to colossal magneto resistance. One of the open question in these compounds is the possible presence of both phases. We have used the ESR technique to look at possible phase separation effects in powdered samples as well as thin films.

9:50 a.m. Dr. S. Zvyagin, National High Magnetic Field Laboratory, Tallahassee
“25 T Millimeter and Sub-millimeter Wave Spectroscopy Facility at the NHMFL”
We report on features of a new high field millimeter and sub-millimeter wave spectroscopy facility at the NHMFL, Tallahassee (DC Field Facility Program)*. Currently, the facility allows for high-resolution electron spin resonance experiments to be carried out in a very wide, quasi-continuously covered frequency range of 140 - 700 GHz (4.6 – 23.3 cm-1), in a fields up to 25 T, and over a range of temperatures from 1.5 to 300 K. A key feature of the spectrometer is the set of easily-tunable, highly-monochromatic, stable, and relatively powerful microwave sources, Backward Wave Oscillators (BWOs). These radiation sources in combination with the highly-homogeneous (12 ppm/1 cm DSV) magnetic field provided by the Keck resistive magnet makes the facility an extremely powerful tool in solving a large number of scientific problems in this frequency-field range, including high-resolution 25 T EPR spectroscopy of transition-metal ions, physics of field-induced and spontaneous phase transitions, antiferromagnetic and cyclotron resonance phenomena, spin-dynamics in low-dimensional and highly correlated electron systems. Some recent results are reported.

10:35 a.m. Coffee Break

11:00 a.m. Dr. B. McCombe, S.U.N.Y. at Buffalo
“Far Infrared Spectroscopy of Donors and Excitons in Semiconductor Quantum Wells in High Magnetic Fields”
I will review recent work at University of Buffalo on confined shallow donors (neutral and charged) and confined magnetoexcitons (neutral and charged) primarily in GaAs/AlGaAs quantum well structures. We have studied internal transitions of quantum-confined (Si) donors in the presence of excess electrons by FTIR transmission spectroscopy and find transitions associated with the negatively charged donor (D-minus), which is blue shifted in the presence of large densities of excess electrons. The blue shift is a signature of a bound (to D+) magnetoplasmon excitation. More recently, we have studied internal transitions of photoexcited neutral and charged magnetoexcitons in similar structures by optically detected resonance (ODR) spectroscopy. ODR spectroscopy is a sensitive technique in which the effects of resonant absorption of far IR laser radiation are detected as small changes in a particular photoluminescence feature. This work includes the first observation of internal transitions of negatively charged excitons (X-minus) and many body effects associated with excess electrons. Due to magnetic translational invariance the allowed transitions for X-minus are bound-to-continuum transitions, in contrast to those for D-minus, which are bound-to-bound transitions. The spectroscopic signatures are qualitatively different. The many body effects are manifest again as a blue shift of the X-minus transitions, and are interpreted as excitations of magnetoplasmons bound to mobile holes.

11:45 a.m. Discussion

12:15 p.m. Lunch Break at workshop location

01:30 p.m. Prof. D. Drew, University of Maryland
“IR Hall Effect in BSCCO: Transport and ARPES Compared”
In the effort to develop an understanding of the cuprates ARPES has provided detailed measurements of the electron spectral density function. This information is the input for any transport theory of these materials. Since BSCCO has been the material of choice for ARPES, it is particularly important to relate the transport properties of BSCCO to these ARPES results. In this talk I will compare the DC and AC (infrared) magneto-conductivity of BSCCO with the ARPES data. The IR measurements are made on cleaved single crystals of thickness ~ 100 nm.

02:15 p.m. Prof. T. Tyson, New Jersey Institute of Technology
“Local Structural Studies of Magnetic Field Induced Melting of the Charged Ordered State in L _0.5Ca _0.5MnO₃”
Systematic high-field bulk magnetization studies of the half doped system, La_0.5Ca_0.5MnO₃, have been carried out by previous researchers [1] resulting in the construction of critical field – temperature phase diagrams. The characteristic feature of these diagrams is that, below the charge ordering temperature, magnetic fields first produce a mixed charge ordered antiferromagnetic (insulating) - ferromagnetic (metallic) state over a broad range of fields. At higher fields the charged state is destroyed. To understand the structural changes occurring, we have performed XAFS measurements to probe the local structure about the Mn and La sites in fields varying from 0 to 10 T. We track the Mn-O and Mn-Mn correlations as well as the La-O distribution as a function of field. We find that with respect the Mn-O and Mn-O-Mn correlations, the AFM to FM transition is much more abrupt than suggested by magnetization measurements. The field-induced meta stable structure at low temperature is also examined. The capabilities of the recently commissioned 10 T x-ray spectroscopy magnet will also be discussed. This work is supported by NSF IMR grant DMR-0083189.

[1] G. Xiao et al, Phys. Rev. B 54, 6073 (1996).

03:00 p.m. Coffee break

03:20 p.m. Prof. V. Kiryukhin, Rutgers University
“X-ray Diffraction Studies of Magnetic Field Effects on Multiphase States in Magnetoresistive Manganites”
Manganite perovskites A_1-xB_xMnO₃ exhibit a number of unique properties, such as the colossal magnetoresistance effect (CMR), and extreme changes in transport and magnetic properties induced by irradiation or application of pressure. It has been shown recently that these effects stem from the intrinsic structural and electronic inhomogeneities, with length scales ranging from microns (martensitic states), to submicron scale (fine mixtures of metallic and insulating states), to nanometers (charge/orbital correlations). In this talk, we give an experimental overview of the role of these inhomogeneities in the CMR phenomenon, concentrating on x-ray diffraction studies in an applied magnetic field. The microscopic mechanism of the CMR effect is discussed, and the important role of nanoscale charge/orbital correlations in the field-induced insulator-metal transitions is demonstrated. Finally, we briefly discuss a new user facility for x-ray diffraction in high magnetic fields which is under development at X21.

04:05 p.m. Discussion

5:00 p.m. Adjourn

Register: Please register for the meeting and workshops (including speakers) at http://www.nsls.bnl.gov/users/meeting/2003/registration/