High Magnetic Field Science and Its Application in the United States: Current Status and Future Directions (2013)

6

Combining High Magnetic
Fields with Scattering and
Optical Probes

In addition to the use of high magnetic fields as discussed in Chapters 2, 3, and 4, these fields can also be used in combination with other tools, such as scattering probes, to study the properties of materials. For instance, understanding the functionality of materials requires a deep and thorough knowledge of the spatial ordering of atoms—their charge, orbitals, and moments—as well as the fluctuations and collective modes of these quantities. This information can be obtained from neutron and X-ray scattering experiments that take place in magnetic fields. Similarly, because magnetic fields affect the electronic and magnetic properties of matter in a precise and reversible way, combining high magnetic fields with optical experiments can be used to explore these properties and how they change. In this chapter, the committee discusses opportunities for new science through scattering and optical probes of materials at high magnetic fields.

BRINGING HIGH MAGNETIC FIELDS TO NEUTRON AND X-RAY SCATTERING FACILITIES

Neutron and X-ray scattering experiments have seen some radical advances since the previous COHMAG review of high field facilities. The most dramatic developments in neutron and X-ray scattering in the United States have been the successful launch of the X-ray free electron laser Linac Coherent Light Source (LCLS) at Stanford Linear Accelerator Center (SLAC) and the pulsed spallation neutron source (SNS) at Oak Ridge, but new opportunities have also been provided by X-ray speckle experiments on magnetic materials, by Resonant Inelastic

(soft) X-ray Scattering (RIXS), pioneered at the Swiss Light Source (SLS), and by the enormously efficient multi-angle crystal spectrometer (MACS) at the National Institute of Standards and Technology (NIST) for neutron spectroscopy. These advances in scattering capabilities need to be complemented by appropriate sample environments, including high-field magnets.

The science drivers that are described elsewhere in this report argue compellingly for the range of novel electronic phenomena and phases that can be found at high fields, and there can be no question that unleashing the full power of scattering techniques is imperative if we are to have the same understanding of these high-field phases that we have of those that are found in zero field. To give some idea of the potential impact that neutron and X-ray scattering tools might have for producing new scientific insight, the committee provides a few examples of the types of experiments that would be made possible with the combination of magnetic fields in the range of 20-40 T and scattering techniques, and also indicates some new opportunities at the interface with the life sciences:

FIGURE 6.1 Inelastic neutron study of magnetic excitations near a quantum critical point induced by an applied magnetic field. By applying a transverse magnetic field to the quasi-one-dimensional Ising spin compound CoNb₂O₆, the system can be driven from a magnetically ordered phase to a nonmagnetic paramagnet. The quantum critical point separating the two phases, predicted to belong to an exotic symmetry class, characterized by the Lie group E₈ familiar to elementary particle theorists, gives rise to a peculiar spectrum of low energy excitations as the field is varied about its critical value. Neutron scattering results, shown in the figure, confirm these predictions. SOURCE: From R. Coldea, D.A. Tennant, E.M. Wheeler, E. Wawrzynska, D. Prabhakaran, M. Telling, K. Habicht, P. Smeibidl, and K. Kiefer, 2010, Quantum criticality in an Ising chain: Experimental evidence for emergent E8 symmetry, Science 327:177-180. Reprinted with permission from AAAS.

While the measurement techniques and scattering facilities have undergone radical improvement since the last report, the maximum magnetic field available to experimenters at these facilities has remained fixed at 16 T. In the same time interval, a new user facility that combines a free electron laser and pulsed fields as large as 85 T has become operational in the Dresden High Magnetic Field Laboratory. A new facility is under construction at Helmholtz Zentrum Berlin (HZB), formerly the Hahn-Meitner Institut in Berlin, that will bring a series-connected hybrid magnet, constructed by the NHMFL in Tallahassee, to a neutron scattering center. Because HZB is a national user facility and has a peer-reviewed, merit-based proposal system, U.S. scientists and engineers can access its neutron measurement capabilities, based on the technical merit of their proposed experiments. When

TABLE 6.1 Magnetic Field Capabilities at Domestic Neutron and Light Sources^a

^a Only magnets whose maximum field equals or exceeds 5 T are listed in this table.

^b Unless otherwise stated, the magnets listed in this table are superconducting.

^c The facilities contacted for this report include those listed above and CHESS (Cornell), NSLS (Brookhaven National Laboratory), and the SSRL at Stanford University. Although most facilities responded, some did not have the magnetic field capabilities that warranted their addition to this list.

the series-connected hybrid magnet becomes operational, U.S. researchers should be able to submit proposals for access to the magnet/neutron scattering facility.

Table 6.1 lists the magnetic fields available at selected U.S. facilities, and Table 6.2 lists international scattering facilities. Most have superconducting magnets, while fewer can provide their users with constant fields as large as 16-17 T. Several have expanded the field range to 30 T and even 50 T using pulsed-field magnets, although this is limited largely to diffraction and perhaps small-angle neutron scattering experiments, except at the most intense sources. Improved pulsed-field magnets are being developed at several neutron sources domestically and internationally.

In view of the considerable new physics obtained from examining transport and bulk properties at the NHMFL and its peers worldwide, for which the corresponding correlation functions and fluctuation spectra are unknown, there is a

TABLE 6.2 Magnetic Field Capabilities at Selected Major International Neutron Sources^a

clear impetus to build higher field capabilities at neutron and X-ray facilities. It is time to harness the developments in superconducting magnet and pulsed-field capabilities that the past decade has brought for scattering science applications. For elastic (diffraction) experiments, which measure order parameters in nonmetallic systems, using pulsed fields will be very informative. On the other hand, for inelastic spectroscopies, which are intrinsically flux-limited, and metallic samples affected by Eddy currents, these capabilities will need to be steady state.

Planning for the Future

There are multiple performance metrics for magnets, and this is especially true for beam experiments. Of particular significance are not only the absolute magnitudes H of the fields attained and the duty cycle (for pulsed-field magnets), but also the H/T ratios, where T is the sample temperature; the direction of the magnetic fields relative to momentum and polarization vectors; apertures and the

NOTE: Many of the facilities that responded to the committee’s requests for information also indicated that they had a number of additional magnets either in procurement or in the development stage. While those magnets are not listed here, the committee is very interested in progress in this area.

^a Only magnets whose maximum field equal or exceed 5 T are listed in this table.

^b Unless otherwise stated, the magnets listed in this table are superconducting.

arrangements of beam windows; sample cooling powers, taking into account the windows and beam heating effects; and mobility of the magnet/sample dewars, which are frequently mounted on the moving stages of spectrometers. RIXS, which depends on soft X-rays, will also demand suitable vacuum environments. Experiments themselves must be designed individually with this multidimensional phase space in mind (Figure 6.2). Their execution is of a difficulty that has prevented even experiments in modest 5 T fields from becoming any more routine than they were 20 years ago. The downgrading of traditional low-temperature physics skills among X-ray and neutron professionals since the era of high-temperature superconductors has also resulted in a skills gap, particularly at synchrotrons.

These considerations, together with the scientific opportunities described earlier, which could be exploited by a more vigorous program of high-field research at neutron and X-ray facilities, demand more than the simple procurement of large magnets for certain beam lines. There are new products such as “dry” dilution refrigerators and superconducting magnets exploiting new materials with higher

FIGURE 6.2 Planning issues for a high-field experiment using neutrons. Frame (a) shows restricted neutron flight paths into and away from sample centered in circle at middle, while frame (b) shows the highly restricted regions, enclosed by the red lines, accessible in reciprocal space for the La₂-_xSr_xCuO₄ superconducting crystal. The blue lines represent where the scattering for magnetic “stripe” order occurs in this crystal. SOURCE: Reprinted by permission from Macmillan Publishers Ltd: Nature Materials. B. Lake, K. Lefmann, N. B. Christensen, G. Aeppli, D. F. McMorrow, H. M. Ronnow, P. Vorderwisch, P. Smeibidl, N. Mangkorntong, T. Sasagawa, M. Nohara and H. Takagi. Three-dimensionality of field-induced magnetism in a high-temperature superconductor. Nature Materials 4:658–662. Copyright 2005.

critical fields, and in vacuo magnet mounts, which effectively convert superconducting magnets into experimental consumables, all of which together will make it much easier to meet the experimental challenges than the currently installed base of magnet cryostats. Much important science would be enabled by the relatively simple and expeditious procurement of numerous modern 10-16 T magnet/cryostat systems for U.S. large facilities, together with the recruitment of low-temperature/high-field specialists from the “small science” communities, which already have significant practical experience with such systems. The resulting activity will then prepare the X-ray and neutron communities to properly manage and exploit more ambitious high-field systems such as hybrid and pulsed-field magnets.

Conclusion: Neutron and X-ray scattering measurements have played a central role in explicating the behaviors of virtually every class of strongly interacting matter. However, there continues to be almost no progress in the United States on bringing higher fields to neutron and X-ray scattering user facilities. It is clear that difficulties in establishing and maintaining an effective steward-partner relationship between scattering facilities and the NHMFL, as well as their respective sponsors, have been a contributing factor to this lack of progress. This is rapidly becoming a lost opportunity for U.S. science, and bold action is needed now to take the lead in this important area.

Recommendation: New types of magnets should be developed and implemented that will enable the broadest possible range of X-ray and neutron scattering measurements in fields in excess of 30 T. This requires as a first step the expeditious procurement of modern 10-16 T magnet/cryostat systems for U.S. facilities, together with the recruitment of low-temperature/ high-field specialists. Second, a 40 T pulsed-field magnet should be developed with a repetition rate of 30 s or less. Third, building on the development of a high-temperature all-superconducting magnet, which was recommended earlier, a wider-bore 40 T superconducting dc magnet should be developed specifically for use in conjunction with neutron scattering facilities. New partnerships among federal agencies, including the Department of Energy, the National Institute of Standards and Technology, and the National Science Foundation, will likely be required to fund and build these magnets, as well as to provide the funds and expertise that will be needed to operate these facilities for users once they are built. (See the discussion in Chapter 9.)

COMBINING OPTICAL PROBES AND HIGH MAGNETIC FIELDS

Magneto-optical experiments provide a powerful set of tools to obtain insights into the properties of materials, and such experiments are often an ideal way to study new physical phenomena. In this section we use magneto-optics as a general description of all experiments probing light-matter interaction over the extended frequency range from microwave frequencies to ultraviolet. Experimentally, light probes the complex refractive index that contains detailed information on low-energy excitations and collective modes in the studied system (Figure 6.3).

In the 1960s, an era when magnetic field research in specialized facilities started, interband and intraband magneto-optical experiments, most prominently at the Francis Bitter Magnet Laboratory in the United States, laid the basis of our knowledge of band structure of most semiconductors and inspired both theory and new experiments. This work evolved into impurity spectroscopy and the study of excitons in a huge number of semiconducting and insulating materials. At the basis of these studies lie the facts that impurity levels show Zeeman-like splitting in magnetic fields, and conduction and valence bands become Landau levels, all with characteristic energies and symmetries specific for the material studied and dictated by band parameters. These early experiments mainly entailed rather straightforward transmission and luminescence measurements, with optical sources for interband studies and infrared (IR) sources for intraband measurements (e.g., cyclotron and spin resonance) (Balkanski, 1994; Landsberg, 1994). This research has significantly contributed to the general understanding of metals and semiconductors and to the basis of much of the electronic and magnetic materials technology commonplace today.

FIGURE 6.3 Characteristic energy scales in solids. NOTE: DFG, difference frequency generation; OPA, optical parametric amplifier; QCL, quantum cascade laser. SOURCE: Adapted with permission from D.N. Basov, R.D. Averitt, D. van der Marel, M. Dressel, and K. Haule, 2011, Electrodynamics of correlated electron materials, Reviews of Modern Physics 83:471, copyrighted by the American Physical Society.

Since the initially available samples of new materials are often of subpar quality, field-induced changes can be observed only at the very high magnetic fields, where magnetic energies exceed the disorder-broadening of the levels. For this reason, new materials are, in many cases, first studied at high magnetic field laboratories, thus making these facilities into antennas for advanced materials and condensed matter research. Magneto-optics or magnetospectroscopy remains a very important part of the work at high-field facilities, which are always focused on the newest materials

or material structures. The set of experimental tools in the field of magneto-optics is being continuously expanded and becoming increasingly sophisticated.

In the 1970s and 1980s, semiconductor heterostructures were at the focus of magneto-spectroscopy research (Balkanski, 1994; Landsberg, 1994; Landwehr and Rashba, 1991). Current work is centered at the forefront of fundamental and applied physics, including zero-dimensional structures (quantum dots, quantum wires), oxide heterostructures, graphene, magnetic semiconductors, correlated electron systems, unconventional high-T_c superconductors, topological insulators, and many other systems. However, not only can material properties be studied but also new physical effects can be directly observed. For instance, the spin texture in semiconductor bilayer systems can be observed from the degree of polarization of transmitted light (Aifer et al., 1996). Also, many-body excitations responsible for the fractional quantum Hall effect can be investigated by Raman scattering experiments that allow probing of both energy and wavevector of the collective excitation (Blokland et al., 2011).

Over time, the performance of laser sources has increased dramatically, covering an ever-wider frequency range and allowing time-resolved studies in the femtosecond (fs) domain. These capabilities have enabled many nonlinear techniques (e.g., higher harmonic generation) to be done at high magnetic fields (Molter et al., 2010). Additionally, detectors have improved tremendously with the advent of charge-coupled device (CCD) arrays, allowing ever-weaker signals to be measured. Moreover, modern CCD detectors in combination with dispersive elements allow an entire high-resolution spectrum to be collected in a single-shot measurement. Finally, the development of low-loss single-mode fibers and compact optical components (lenses, polarizers, even nanometer-resolution translation stages) have allowed scientists to perform very advanced optical experiments at the highest possible magnetic fields and even at very low temperatures. For instance, single-object spectroscopy (single molecules, single quantum dots, nanowires) using confocal microscopy at very high fields with a subwavelength resolution are now possible (Htoon et al., 2009) and provide great promise for future discoveries. The use of single objects instead of ensembles of many dots has the enormous advantage in that spectral linewidth broadening due to unavoidable inhomogeneity of an ensemble of dots or molecules is eliminated, which increases resolution by orders of magnitude. Good examples are colloidal nanodots, made from II-VI elements by purely chemical means (Blokland et al., 2011). These dots may serve as coatings in LED sources or on top of solar cells, which convert the incoming solar photon energies to lower ones through absorption and re-emission in a very efficient way. Developing such an application requires a detailed knowledge of the electronic structure in these dots, and magneto-spectroscopy has contributed significantly to this crucial knowledge (Blokland et al., 2011).

Optical techniques in the past have had their greatest impact in advancing the

physics of semiconducting materials. However, with nonlinear techniques becoming available due to better equipment, metallic materials can now be studied as well. Such studies are still in a relatively early phase but hold great promise for the future. Current active research in this vein includes small metallic particles and graphene microcrystals, both of which are of great interest in the areas of nanophotonics and nanoplasmonics (Crassee et al., 2012). The use of high magnetic fields in combination with advanced optical spectroscopy has therefore the potential of making great contributions to both science and technology. As mentioned before, a pioneering role is expected for dedicated high-field laboratories, since the highest fields are needed to provide the clearest data. Of particular interest here is that the quantum-mechanical magnetic length at high magnetic fields is in the nanometer range, so the interplay of magnetic fields and size effects in nanoscale structures may open new research areas.

In addition, many physical phenomena depend explicitly on the presence of a high magnetic field. Thus, spectroscopic studies of field-induced metal-insulator transitions, of the normal state of high-T_c superconductors, and of boundaries between different magnetic states in correlated electron systems are highly desirable.

Magneto-Optical Experiments in the Far Infrared

In the terahertz and far IR region (0.5-25 THz), progress has been more modest than in the mid-infrared and visible regime, largely because sources, detectors, and optical components here are far less developed. Yet, from a scientific viewpoint this energy region in high magnetic fields is of particular importance, since it covers the magnetic resonances (spin resonance, antiferromagnetic resonance, magnons, cyclotron resonance, and other important effects) in fields between 20 and 100 T (Figure 6.3). Recently completed IR magneto-optical studies of high-T_c superconductors (LaForge et al., 2008; Basov and Timusk, 2005) graphene (Jiang et al., 2007; Orlita and Potemski, 2010) and topological insulators (Valdés Aguilar et al., 2012; Schafgans et al., 2012) attest to unique capabilities of spectroscopic studies in far-IR and THz ranges. Phonon and molecular vibration energies are typically in this regime, allowing for the study of many types of coupled modes. Far IR radiation is of particular importance because its low energies (meV range) are such that electronic and magnetic states are probed very near their equilibrium state (measuring essentially ground state properties). This is a significant advantage compared to visible range optical spectroscopy, where excited states usually are studied. For all of these reasons, the Dresden facility and the Nijmegen high magnetic field laboratory of the EMFL have integrated free electron lasers (FELs), providing very high intensity, quasi-monochromatic, pulsed or quasi-continuous radiation, in exactly this frequency range, at their facilities. The much higher power of these sources (a 10⁴- to 10⁶-fold increase compared to traditionally available sources) and their

tunability (instead of fixed frequencies with backward wave oscillators or molecular lasers) will lead to a wide range of new experiments hitherto impossible. Among them will be mode-selective excitation of phonons in correlated metal oxides triggering the transition from insulating to metallic state (Rini et al., 2007).

The NHMFL in Tallahassee has also recently put forward a proposal to combine its magnets with FEL dedicated sources. Serious consideration should be given to this provision of tunable radiation for pump-probe measurements in high magnetic fields at NHMFL. The NHMFL proposal presented to the committee is considerably more elaborate than the European FELs and would cover a broader range of wavelengths. But the estimated construction¹ and operating costs of this facility are significant, so less costly alternatives should be explored. A systematic approach to complete coverage of the terahertz regime by alternative methods, including, for example, a suite of quantum cascade lasers, should be explored, although it may be difficult to compete with FELs in optical field strength, wavelength, and time structure. Given the interest beyond the high-field community in such coverage, the committee suggests partnering with other agencies, including particularly those sponsored by the DOE or DOD. Another option that should be considered is to bring the high magnetic field capabilities to a source of terahertz radiation. A potentially cost-effective approach, one that would meet the needs of the scientific community, is to locate a moderately high field (commercially available, 10-20 T, all-superconducting) magnet at a centralized, tunable source of terahertz radiation. Although the very highest fields achievable at NHFML would not be reached at a centralized FEL facility, a significant breadth of scientific phenomena (including magnetic excitations, collective modes in correlated electron systems, lattice vibrations, and band gaps) could be studied even at lower fields.

Apart from FEL’s recent developments in the ultrafast regime, nonlinear optics now allow one to generate intense THz fields using much more modestly priced tabletop setups (Yeh et al., 2007; Hirori et al., 2011). The attainable fields can exceed 1 MV/cm, which is sufficient to switch between the insulating and metallic state of correlated transition metal oxides (Liu et al., 2012). The anticipated progress in the technology of solid-state lasers directed towards enhancing spectroscopic and pump-probe capabilities will enable a broad range of breakthrough magneto-optics studies of new materials over the next decade.

At the high power densities enabled by state-of-the art laser sources, it is possible to break up Cooper pairs in films of high-T_c superconductors, meaning the phase transition line in the T_c-H_c plane can be spectroscopically studied. Furthermore, these power densities are sufficient to induce phase transitions, including superconducting transitions in nonsuperconducting compounds (Fausti et al.,

________________

¹ A cost estimate ($86 million) from 2008 for construction was presented to the committee by Greg Boebinger, NHMFL, on March 12, 2012.

2011). Magneto-spectroscopy offers tremendous opportunities to investigate the physics of these new states of matter. Novel spectroscopic capabilities when combined with ultrahigh magnetic fields will additionally enable cyclotron resonance studies of heavy Fermion systems (Dordevic et al., 2006) or even more complicated correlated electron systems.

Among other applications of high-power-density radiation sources in high magnetic field experiments, one may think of pulse probe experiments in the far IR, sum frequency and second harmonic generation, and spin-echo electron paramagnetic resonance at frequencies and field ranges at least 10 times higher than is possible now. Furthermore, the combination of far-IR radiation with scanning probe techniques that can position metallic nanometer-sized antennas with subwavelength precision may even allow far-IR spectroscopy of single objects. Continuous wave far-IR sources, together with high-field NMR, may also be used to create dynamic nuclear polarization, whereby the nuclear spin population is polarized by coupling to a set of electron spins that are polarized using far-IR light at terahertz frequencies. This technique promises orders-of-magnitude-enhanced sensitivity in NMR experiments.

As pulsed magnetic fields will always provide researchers with the highest fields, it is important to improve the set of optical characterization tools compatible with the stringent constraints of these experiments. Certain experiments already can be performed in ultrahigh (170 T) pulsed fields (Booshehri et al., 2012). Nevertheless, subsecond timescales are fundamentally insufficient to acquire quality spectroscopic data in the terahertz and far-IR ranges. Therefore, one can anticipate that the most significant breakthrough results will be obtained in dc fields enabling ample time for data acquisition. Furthermore, in order to take full advantage of IR/optical studies in high magnetic fields, it is imperative to be able to analyze the polarization state of transmitted, reflected, or scattered radiation. This latter requirement presents a significant challenge for broadband magnetospectroscopy. An ideal implementation of optical access to a sample in high magnetic field implies propagation of radiation in free space as opposed to fibers and waveguides.

A combination of far-IR radiation with scanning probe techniques that can position metallic nanometer-sized antennas with subwavelength precision enables infrared spectroscopy and imaging at the nanoscale (Bonnell et al., 2012). Extension of these studies to behavior in high magnetic fields would be quite interesting; however, extension to even to modest dc magnetic fields presents a considerable experimental challenge.

In summary, optical and far-IR spectroscopy with high magnetic fields present an ideal combination of experimental techniques that can probe and elucidate properties of the most advanced materials (see Figure 6.4). Furthermore, these techniques provide complementary information to the more commonly used

FIGURE 6.4 Typical energies, in terahertz, of various physical phenomena in fields up to 45 tesla (T), showing that the combination of intense far-infrared generated with free electron lasers combined with high magnetic fields has great promise for innovative research.

measurements of transport, magnetization, and thermodynamic properties. Any high magnetic field facility should have a strong program in this area.

Finding: Photons are one of the most important probes of high-field phenomena, ranging from magnetic resonance, which is of importance to all disciplines, including biology, chemistry, and physics, to excitation spectra in quantum solids, a central theme of modern condensed matter physics. Over the last decade, the use of photons for control as well as diagnostics of matter has become a major theme for condensed matter physics, and the committee expects that experiments entailing such control will place new demands on high field facilities. Photon sources in the frequency range 0.5 to 25 THz are of special importance for high-field experiments, as typical frequencies of electron spin resonances and cyclotron resonances fall in this range for fields between 20 and 100 T.

Recommendation: A full photon spectrum, covering at least all of the energies (from radio-frequency to far infrared) associated with accessible fields,

should be available for use with high magnetic fields for diagnostics and control. At any point in the spectrum, transform-limited pulses of variable amplitude, allowing access to linear and nonlinear response regimes, should be provided. Consideration should be given to a number of different options, including (1) providing a low-cost spectrum of terahertz radiation sources at the NHMFL, (2) construction of an appropriate free electron laser (FEL) at NHFML, or (3) providing an all-superconducting, high-field magnet at a centralized FEL facility with access to the terahertz radiation band.

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