- Email: [email protected]
High magnetic fields and their laboratories
Physica B 177 (1992) North-Holland
22-26
High magnetic fields and their laboratories J.C.
Maana
and P. Wyderasb
“Hochfeld-Magnetlabor, Max-Planck-Znstitut fz’ir Festkiirperforschung, BP 156X, F-38042 Grenoble Cedex, France bService National des Champs Intenses, Centre National de la Recherche Scientifique, F-38042 Grenoble Cedex, France
Some general are given.
considerations
about
the nature
and the role of high magnetic
In this contribution we want to analyse on a more global level the role that high magnetic fields play in the context of condensed matter research, and to derive some organisational implications from this analysis. We pursue to make an assessment as realistic as possible of the various aspects of this question and we try not to be carried away by our enthousiasm for physics in high magnetic fields. A high magnetic field is an experimental parameter able to change the physical state of matter. More specifically, it changes the orientation of angular momenta (spin), quantizes the energy, introduces a new force (the Lorentz force) acting on charged particles which leads to a quantization of their motion, introduces a new length scale (cyclotron or Larmor radius), destroys time-reversal symmetry, and changes the density of states, without adding extra energy to a system. In this broad sense magnetic fields are comparable to other experimental techniques like high pressures and low temperatures, which also change the state of the system, although in a very different way. Each of these techniques has its unique effects which makes them useful for particular classes of experiments. Roughly speaking a field has two possible applications. On the one hand the effect of the field on electronic properties may serve as a technique to obtain more information about zero field properties. For instance cyclotron resonance measures the effective mass; the de Haas0921-4526/92/$05.00
0
1992 - Elsevier
Science
Publishers
field research
in condensed
matter
physics
van Alphen effect, the Fermi surface, etc. On the other hand the magnetic field may induce new phenomena, absent without magnetic field. The quantum Hall effect [l], the fractional quantum Hall effect [2], the field induced Wigner solid [3], etc., are examples of the second type. In this respect there is also a strong analogy with low temperatures or high pressures where a similar distinction can often be made. For instance the temperature dependence of the conductivity reveals the different mechanisms which are responsible for the high temperature conductivity which may serve as an example of the first type; new states like superconductivity or superfluidity are examples of the second type. There exists therefore an important analogy between magnetic fields and the other experimental conditions mentioned before. However there is one very important difference which is that the cost of the production of high fields (>20 T) is between one and two orders of magnitude higher than that of an average well-equipped low temperature laboratory. For instance our French-German Grenoble High Magnetic Field Laboratory could recently renew its 18 years old installation at the total price of about 13 million ECU. More or less these same plans but on a much bigger scale will be realized in the USA in Tallahassee, where the new American National High Magnetic Field Laboratory will be constructed over a period of five years with a grant of more than 130 million ECU. Similarly,
B.V. All rights
reserved
J.C.
Maan, P. Wyder I High magnetic fields and laboratories
in Japan at the Tsukuba High Magnetic Field Laboratory in recent years about 65 million ECU per year have been invested. Although there exist smaller very good high field laboratories in the world, it should be emphasized that only three big laboratories worldwide are a strict minimum for this type of competitive research; the difference of an order of magnitude between the American and the Japanese efforts compared with the European investment is also worthwhile noting. The prices to pay imply almost always an engagement on a national or even international level. Therefore, moneywise high field facilities are becoming more similar to other big installations like neutron reactors or synchrotron radiation facilities, although they are typically still several times less expensive. For reasons to be explained later it may even be expected that to reach fields higher than those presently available, even bigger investments will be needed, and this implies the necessity to present convincing arguments to justify such expenses. The very nature of high magnetic fields makes that the argumentation for the funding of such an expensive facility is inherently more indirect than that for instance for a synchrotron. Synthrotrons are designed to provide spectroscopically precise answers to precise questions. This is because, contrary to high fields, such machines do not change the state of matter, instead they study a material in some specific conditions. Therefore, given the specifications of the machine it is possible to sketch the sort of questions that can be studied. We mentioned before the two possible uses of fields. In the sense of using fields as a trick to obtain information of the zero field condition, the technique has a strong similarity to that of a synchrotron; to some extend the systems and questions studied can be identified. However in its second use, namely to create new conditions, it is not known whether new phenomena will be found, and in fact new discoveries may be done by chance, albeit guided by intuition. It is generally believed that other highly exciting and interesting effects will occur in the 50T-range, although at present there exist only suggestive hints for this belief. This second use is maybe the more interesting and stimulating one, but at the
23
same time the most vague to justify; new discoveries cannot be planned. The reason for the high price of high fields lies in the technological problems in their production. For illustration we show in fig. 1 the evolution of the maximum obtained DC field as a function of time. It can be seen that progress is slow. The doubling time for the maximum field is about 25 years, and by extrapolation DC fields of around 50 T may be attained at the change of the century. Present state of the art magnet-design has exploited up to the limits the geometry of the current distribution and the cooling in the coils. A theoretical optimization of these parameters gives that given the electrical and mechanical properties of copper, given 10 MW of available power, and given the inside bore of the coil (50 mm) a maximum field of 26T [4] can be obtained. The actual polyhelix coil in our laboratory [5] produces 25 T, which is very close to the theoretical limit. Therefore, unless a revolutionary new material with better mechanical and electrical properties than copper is discovered, higher fields can only be obtained by using more and a new power. Using hybrid technology, power supply of 20 MW (being built in our laboratory and in Tallahassee, USA) eventually fields of around the afore-mentioned 50 T could probably be reached. Similar projects are also under way in Japan in Tsukuba’s high magnetic field laboratory. More power, more complex
0 u,
50.
Q a, L_
Magnet .
Hybrid Polyhelix
L
CONI
l
0 0
.
10
Bitter
cotI
Supercond magnet
.9
5-
11900 I
. Air cored
1920
1940
coil
1960
1980
2(
Year
Fig. 1. Evoiution of the maximum attained magnetic field values as a function of time. Resistive coils (closed circles) and superconducting coils (open circles).
24
J.C.
Maan, P. Wyder I High magnetic fields and laboratories
magnets, higher superconducting coil base-fields, all imply higher financial investments. Even if superconducting wires with much higher critical fields would be available, it is not a priori clear that this would be much more economical. One of the main limitations on the maximum field values are the mechanical stresses which are equally important for superconducting magnets. In this paper we have concentrated mainly on steady magnetic fields. However,,it is very much worthwhile to note that pulsed quasistationary fields values around 100 T for one second could realistically be envisaged. Experimentally, one second is a rather long time and many interesting experiments could be done in such an installation. It may be expected that those quasi-stationary fields will constitute the long term future for high magnetic fields. However, these installations with long pulses are definitely not less costly than the steady state ones. On the other hand, with shorter pulses even higher fields can be obtained and these may lead the way. Short field pulses can be realized on a scale which is affordable for small laboratories and universities, although due to the short times these installations have a more limited range of experimental possibilities. I.e. there is the need for all these types of installations. Despite all these efforts, it has to be realized that in many respects fields now attainable in the laboratory are never really high compared with relevant physical quantities. A Bohr magneton placed in a magnetic field of 1 T has an energy of 0.058 meV or 0.67 K, which is very little compared to room temperature. Similarly the size of a cyclotron orbit in the same field is 25 nm which is very large to most naturally occurring lengths such as a typical lattice spacing (0.5 nm) or the Bohr radius (0.05 nm). It is therefore clear that interesting new physical properties can only be found in judiciously chosen experiments, chosen in such a way as to obtain extreme physical conditions. It is worthwhile noting that the same is true also for low temperatures: only 3He and 4He become superfluid, not all metals become superconducting. The previous paragraphs could seem to suggest that the future of high field research is
rather gloomy. This impression is certainly wrong as the interest in high fields is definitely growing in the scientific community in the last decades. To illustrate this point quantitatively we show in fig. 2(a) the evolution of the total number of publications in physics and those publications which contain the words “high magnetic field(s)” in the abstract or the title. Whereas the total number of publications increases linearly with time, those concerning fields increase superlinear. This trend is exemplified in fig. 2(b), where the average fraction of publications with
280
l Total
number
q Publications
of
publications/l000
with
Year l Total
number
of
publications/l000
51”“1”“1”“1”“,““,‘,,, 65 70 75 80
b 85
90
G
Year Fig. 2. (a) Evolution of the total number of publications the physics literature (closed circles, in units of 1000) and those publications containing the words “high magnetic field(s)” in the abstract or the title (open squares). The drawn lines represent respectively a linear and a quadratic regression fit to the data; (b) The fraction of the number of publications containing “high magnetic fields” to the total number of publications. The drawn straight line is a linear regression fit.
J. C. Maan, P. Wyder I High magnetic fields and laboratories
25
26
J. C. Maan, P. Wyder I High magnetic fields and laboratories
chosen systems are actually promising to do in high fields. Only people having an active interest in high field research will care to identify and actively look for those systems. Only if sufficient new results, identifiable as originating from a high field laboratory are published it will be possible to convince the scientific community to investm this type of activity. At this moment there exists a remarkably favourable climate for the simulation of high field research all over the world. The Tallahassee National High Magnetic Field Laboratory, the Tsukuba High Magnetic Field Laboratory and the Grenoble High Magnetic Field Laboratory, all have received very substantial grants which will allow the building of new installations or the substantial upgrading of existing ones. This funding has been granted despite the fact that progress in magnet technology is slow and that attainable fields are not really high. In fact this new financial injection is, above all, the result of a very active magnetic-field-users community which has produced an appreciable amount of
sound, interesting and exciting experimental results which appeal to the scientific community. This last argument proves in our opinion once more the point that we wanted to make, namely: although one must try to obtain the highest possible fields, it is not so much the record field which make a laboratory successful but it is rather the quality of the scientific output.
References [l] K. von Klitzing, G. Dorda and M. Pepper, Phys. Rev. Lett. 45 (1980) 494. [2] D.C. Tsui, H.L. Stormer and A.C. Gossard, Phys. Rev. Lett. 48 (1982) 1559. [3] F.I.B. Williams, P.A. Wright, R.G. Clark, E.Y. Andrei, G. Deville, D.C. Glattli, 0. Probst, B. Etienne, C. Dorin, CT. Foxon and J.J. Harris, Phys. Rev. Lett. 66 (1991) 3285. [4] G. Aubert, private communciations and to be published. [5] H.-J. Schneider-Muntau, IEEE Trans. Magn. MAG-18 (1982) 1565. [6] C. Kittel, Introduction to Solid State Physics, 6th and 3rd Ed. (Wiley, New York, 1986; 1966).
22-26
High magnetic fields and their laboratories J.C.
Maana
and P. Wyderasb
“Hochfeld-Magnetlabor, Max-Planck-Znstitut fz’ir Festkiirperforschung, BP 156X, F-38042 Grenoble Cedex, France bService National des Champs Intenses, Centre National de la Recherche Scientifique, F-38042 Grenoble Cedex, France
Some general are given.
considerations
about
the nature
and the role of high magnetic
In this contribution we want to analyse on a more global level the role that high magnetic fields play in the context of condensed matter research, and to derive some organisational implications from this analysis. We pursue to make an assessment as realistic as possible of the various aspects of this question and we try not to be carried away by our enthousiasm for physics in high magnetic fields. A high magnetic field is an experimental parameter able to change the physical state of matter. More specifically, it changes the orientation of angular momenta (spin), quantizes the energy, introduces a new force (the Lorentz force) acting on charged particles which leads to a quantization of their motion, introduces a new length scale (cyclotron or Larmor radius), destroys time-reversal symmetry, and changes the density of states, without adding extra energy to a system. In this broad sense magnetic fields are comparable to other experimental techniques like high pressures and low temperatures, which also change the state of the system, although in a very different way. Each of these techniques has its unique effects which makes them useful for particular classes of experiments. Roughly speaking a field has two possible applications. On the one hand the effect of the field on electronic properties may serve as a technique to obtain more information about zero field properties. For instance cyclotron resonance measures the effective mass; the de Haas0921-4526/92/$05.00
0
1992 - Elsevier
Science
Publishers
field research
in condensed
matter
physics
van Alphen effect, the Fermi surface, etc. On the other hand the magnetic field may induce new phenomena, absent without magnetic field. The quantum Hall effect [l], the fractional quantum Hall effect [2], the field induced Wigner solid [3], etc., are examples of the second type. In this respect there is also a strong analogy with low temperatures or high pressures where a similar distinction can often be made. For instance the temperature dependence of the conductivity reveals the different mechanisms which are responsible for the high temperature conductivity which may serve as an example of the first type; new states like superconductivity or superfluidity are examples of the second type. There exists therefore an important analogy between magnetic fields and the other experimental conditions mentioned before. However there is one very important difference which is that the cost of the production of high fields (>20 T) is between one and two orders of magnitude higher than that of an average well-equipped low temperature laboratory. For instance our French-German Grenoble High Magnetic Field Laboratory could recently renew its 18 years old installation at the total price of about 13 million ECU. More or less these same plans but on a much bigger scale will be realized in the USA in Tallahassee, where the new American National High Magnetic Field Laboratory will be constructed over a period of five years with a grant of more than 130 million ECU. Similarly,
B.V. All rights
reserved
J.C.
Maan, P. Wyder I High magnetic fields and laboratories
in Japan at the Tsukuba High Magnetic Field Laboratory in recent years about 65 million ECU per year have been invested. Although there exist smaller very good high field laboratories in the world, it should be emphasized that only three big laboratories worldwide are a strict minimum for this type of competitive research; the difference of an order of magnitude between the American and the Japanese efforts compared with the European investment is also worthwhile noting. The prices to pay imply almost always an engagement on a national or even international level. Therefore, moneywise high field facilities are becoming more similar to other big installations like neutron reactors or synchrotron radiation facilities, although they are typically still several times less expensive. For reasons to be explained later it may even be expected that to reach fields higher than those presently available, even bigger investments will be needed, and this implies the necessity to present convincing arguments to justify such expenses. The very nature of high magnetic fields makes that the argumentation for the funding of such an expensive facility is inherently more indirect than that for instance for a synchrotron. Synthrotrons are designed to provide spectroscopically precise answers to precise questions. This is because, contrary to high fields, such machines do not change the state of matter, instead they study a material in some specific conditions. Therefore, given the specifications of the machine it is possible to sketch the sort of questions that can be studied. We mentioned before the two possible uses of fields. In the sense of using fields as a trick to obtain information of the zero field condition, the technique has a strong similarity to that of a synchrotron; to some extend the systems and questions studied can be identified. However in its second use, namely to create new conditions, it is not known whether new phenomena will be found, and in fact new discoveries may be done by chance, albeit guided by intuition. It is generally believed that other highly exciting and interesting effects will occur in the 50T-range, although at present there exist only suggestive hints for this belief. This second use is maybe the more interesting and stimulating one, but at the
23
same time the most vague to justify; new discoveries cannot be planned. The reason for the high price of high fields lies in the technological problems in their production. For illustration we show in fig. 1 the evolution of the maximum obtained DC field as a function of time. It can be seen that progress is slow. The doubling time for the maximum field is about 25 years, and by extrapolation DC fields of around 50 T may be attained at the change of the century. Present state of the art magnet-design has exploited up to the limits the geometry of the current distribution and the cooling in the coils. A theoretical optimization of these parameters gives that given the electrical and mechanical properties of copper, given 10 MW of available power, and given the inside bore of the coil (50 mm) a maximum field of 26T [4] can be obtained. The actual polyhelix coil in our laboratory [5] produces 25 T, which is very close to the theoretical limit. Therefore, unless a revolutionary new material with better mechanical and electrical properties than copper is discovered, higher fields can only be obtained by using more and a new power. Using hybrid technology, power supply of 20 MW (being built in our laboratory and in Tallahassee, USA) eventually fields of around the afore-mentioned 50 T could probably be reached. Similar projects are also under way in Japan in Tsukuba’s high magnetic field laboratory. More power, more complex
0 u,
50.
Q a, L_
Magnet .
Hybrid Polyhelix
L
CONI
l
0 0
.
10
Bitter
cotI
Supercond magnet
.9
5-
11900 I
. Air cored
1920
1940
coil
1960
1980
2(
Year
Fig. 1. Evoiution of the maximum attained magnetic field values as a function of time. Resistive coils (closed circles) and superconducting coils (open circles).
24
J.C.
Maan, P. Wyder I High magnetic fields and laboratories
magnets, higher superconducting coil base-fields, all imply higher financial investments. Even if superconducting wires with much higher critical fields would be available, it is not a priori clear that this would be much more economical. One of the main limitations on the maximum field values are the mechanical stresses which are equally important for superconducting magnets. In this paper we have concentrated mainly on steady magnetic fields. However,,it is very much worthwhile to note that pulsed quasistationary fields values around 100 T for one second could realistically be envisaged. Experimentally, one second is a rather long time and many interesting experiments could be done in such an installation. It may be expected that those quasi-stationary fields will constitute the long term future for high magnetic fields. However, these installations with long pulses are definitely not less costly than the steady state ones. On the other hand, with shorter pulses even higher fields can be obtained and these may lead the way. Short field pulses can be realized on a scale which is affordable for small laboratories and universities, although due to the short times these installations have a more limited range of experimental possibilities. I.e. there is the need for all these types of installations. Despite all these efforts, it has to be realized that in many respects fields now attainable in the laboratory are never really high compared with relevant physical quantities. A Bohr magneton placed in a magnetic field of 1 T has an energy of 0.058 meV or 0.67 K, which is very little compared to room temperature. Similarly the size of a cyclotron orbit in the same field is 25 nm which is very large to most naturally occurring lengths such as a typical lattice spacing (0.5 nm) or the Bohr radius (0.05 nm). It is therefore clear that interesting new physical properties can only be found in judiciously chosen experiments, chosen in such a way as to obtain extreme physical conditions. It is worthwhile noting that the same is true also for low temperatures: only 3He and 4He become superfluid, not all metals become superconducting. The previous paragraphs could seem to suggest that the future of high field research is
rather gloomy. This impression is certainly wrong as the interest in high fields is definitely growing in the scientific community in the last decades. To illustrate this point quantitatively we show in fig. 2(a) the evolution of the total number of publications in physics and those publications which contain the words “high magnetic field(s)” in the abstract or the title. Whereas the total number of publications increases linearly with time, those concerning fields increase superlinear. This trend is exemplified in fig. 2(b), where the average fraction of publications with
280
l Total
number
q Publications
of
publications/l000
with
Year l Total
number
of
publications/l000
51”“1”“1”“1”“,““,‘,,, 65 70 75 80
b 85
90
G
Year Fig. 2. (a) Evolution of the total number of publications the physics literature (closed circles, in units of 1000) and those publications containing the words “high magnetic field(s)” in the abstract or the title (open squares). The drawn lines represent respectively a linear and a quadratic regression fit to the data; (b) The fraction of the number of publications containing “high magnetic fields” to the total number of publications. The drawn straight line is a linear regression fit.
J. C. Maan, P. Wyder I High magnetic fields and laboratories
25
26
J. C. Maan, P. Wyder I High magnetic fields and laboratories
chosen systems are actually promising to do in high fields. Only people having an active interest in high field research will care to identify and actively look for those systems. Only if sufficient new results, identifiable as originating from a high field laboratory are published it will be possible to convince the scientific community to investm this type of activity. At this moment there exists a remarkably favourable climate for the simulation of high field research all over the world. The Tallahassee National High Magnetic Field Laboratory, the Tsukuba High Magnetic Field Laboratory and the Grenoble High Magnetic Field Laboratory, all have received very substantial grants which will allow the building of new installations or the substantial upgrading of existing ones. This funding has been granted despite the fact that progress in magnet technology is slow and that attainable fields are not really high. In fact this new financial injection is, above all, the result of a very active magnetic-field-users community which has produced an appreciable amount of
sound, interesting and exciting experimental results which appeal to the scientific community. This last argument proves in our opinion once more the point that we wanted to make, namely: although one must try to obtain the highest possible fields, it is not so much the record field which make a laboratory successful but it is rather the quality of the scientific output.
References [l] K. von Klitzing, G. Dorda and M. Pepper, Phys. Rev. Lett. 45 (1980) 494. [2] D.C. Tsui, H.L. Stormer and A.C. Gossard, Phys. Rev. Lett. 48 (1982) 1559. [3] F.I.B. Williams, P.A. Wright, R.G. Clark, E.Y. Andrei, G. Deville, D.C. Glattli, 0. Probst, B. Etienne, C. Dorin, CT. Foxon and J.J. Harris, Phys. Rev. Lett. 66 (1991) 3285. [4] G. Aubert, private communciations and to be published. [5] H.-J. Schneider-Muntau, IEEE Trans. Magn. MAG-18 (1982) 1565. [6] C. Kittel, Introduction to Solid State Physics, 6th and 3rd Ed. (Wiley, New York, 1986; 1966).