From DOE’s Brookhaven National Laboratory (US) and Paul Scherrer Institute [Paul Scherrer Institut] (CH) : “Understanding the Physics in New Metals”

From DOE’s Brookhaven National Laboratory (US)


Paul Scherrer Institute [Paul Scherrer Institut] (CH)

July 19, 2021

Barbara Vonarburg, Paul Scherrer Institute

Brookhaven Lab Scientist Jonathan Pelliciari now works as a beamline scientist at the National Synchrotron Light Source II (NSLS-II)[below], where he continues to use inelastic resonant x-ray scattering to study quantum materials such as correlated metals.

Researchers from the Paul Scherrer Institute PSI and the Brookhaven National Laboratory (BNL), working in an international team, have developed a new method for complex X-ray studies that will aid in better understanding so-called correlated metals. These materials could prove useful for practical applications in areas such as superconductivity; data processing; and quantum computers. Today the researchers present their work in the journal Physical Review X.

In substances such as silicon or aluminium, the mutual repulsion of electrons hardly affects the material properties. Not so with so-called correlated materials, in which the electrons interact strongly with one another. The movement of one electron in a correlated material leads to a complex and coordinated reaction of the other electrons. It is precisely such coupled processes that make these correlated materials so promising for practical applications, and at the same time so complicated to understand.

Strongly correlated materials are candidates for novel high-temperature superconductors, which can conduct electricity without loss and which are used in medicine, for example, in magnetic resonance imaging. They also could be used to build electronic components, or even quantum computers, with which data can be more efficiently processed and stored.

“Strongly correlated materials exhibit a wealth of fascinating phenomena,” says Thorsten Schmitt, head of the Spectroscopy of Novel Materials Group at PSI: “However, it remains a major challenge to understand and exploit the complex behaviour that lies behind these phenomena.” Schmitt and his research group tackle this task with the help of a method for which they use the intense and extremely precise X-ray radiation from the Swiss Light Source SLS at PSI.

Swiss Light Source SLS Paul Scherrer Institut (PSI)

This modern technique, which has been further developed at PSI in recent years, is called resonant inelastic X-ray scattering, or RIXS for short.

Thorsten Schmitt at the experiment station of the Swiss Light Source SLS, which provided the X-ray light used for the experiments. Credit: Mahir Dzambegovic/Paul Scherrer Institute.

X-rays excite electrons

With RIXS, soft X-rays are scattered off a sample. The incident X-ray beam is tuned in such a way that it elevates electrons from a lower electron orbital to a higher orbital, which means that special resonances are excited. This throws the system out of balance. Various electrodynamic processes lead it back to the ground state. Some of the excess energy is emitted again as X-ray light. The spectrum of this inelastically scattered radiation provides information about the underlying processes and thus on the electronic structure of the material.

“In recent years, RIXS has developed into a powerful experimental tool for deciphering the complexity of correlated materials,” Schmitt explains. When used to investigate correlated insulators in particular, it works very well. Up to now, however, the method has been unsuccessful in probing correlated metals. Its failure was due to the difficulty of interpreting the extremely complicated spectra caused by many different electrodynamic processes during the scattering. “In this connection collaboration with theorists is essential,” explains Schmitt, “because they can simulate the processes observed in the experiment.”

Calculations of correlated metals

This is a specialty of theoretical physicist Keith Gilmore, formerly of the Brookhaven National Laboratory (BNL) in the USA and now at the Humboldt University of Berlin [Humboldt-Universität zu Berlin] (DE). “Calculating the RIXS results for correlated metals is difficult because you have to handle several electron orbitals, large bandwidths, and a large number of electronic interactions at the same time,” says Gilmore. Correlated insulators are easier to handle because fewer orbitals are involved; this allows model calculations that explicitly include all electrons. To be precise, Gilmore explains: “In our new method of describing the RIXS processes, we are now combining the contributions that come from the excitation of one electron with the coordinated reaction of all other electrons.”

To test the calculation, the PSI researchers experimented with a substance that BNL scientist Jonathan Pelliciari had investigated in detail as part of his doctoral thesis at PSI: barium-iron-arsenide. If you add a specific amount of potassium atoms to the material, it becomes superconducting. It belongs to a class of unconventional high-temperature iron-based superconductors that are expected to provide a better understanding of the phenomenon. “Until now, the interpretation of RIXS measurements on such complex materials has been guided mainly by intuition. Now these RIXS calculations give us experimenters a framework that enables a more practical interpretation of the results. Our RIXS measurements at PSI on barium-iron-arsenide are in excellent agreement with the calculated profiles,” Pelliciari says.

Combination of experiment and theory

In their experiments, the researchers investigated the physics around the iron atom. “One advantage of RIXS is that you can concentrate on a specific component and examine it in detail for materials that consist of several elements,” Schmitt says. The well-tuned X-ray beam causes an inner electron in the iron atom to be elevated from the ground state in the core level to the higher energy valence band, which is only partially occupied. This initial excitation of the core electron can cause further secondary excitations and trigger many complicated decay processes that ultimately manifest themselves in spectral satellite structures. (See graphic.)

The graphic shows how an electron (blue dot) can be elevated to different energy levels (dotted arrows) or falls back to lower energy levels. Between the highest energy level and somewhat lower level, secondary processes take place. The curve in the background represents the iron electronic levels.
Credit: Keith Gilmore/Paul Scherrer Institute.

Since the contributions of the many reactions are sometimes small and close to one another, it is difficult to find out which processes actually took place in the experiment. Here the combination of experiment and theory helps. “If you have no theoretical support for difficult experiments, you cannot understand the processes, that is, the physics, in detail,” Schmitt says. The same also applies to theory: “You often don’t know which theories are realistic until you can compare them with an experiment. Progress in understanding comes when experiment and theory are brought together. This descriptive method thus has the potential to become a reference for the interpretation of spectroscopic experiments on correlated metals.”

See the full article here .


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The Paul Scherrer Institute [Paul Scherrer Institut] (CH) is the largest research institute for natural and engineering sciences within Switzerland. We perform world-class research in three main subject areas: Matter and Material; Energy and the Environment; and Human Health. By conducting fundamental and applied research, we work on long-term solutions for major challenges facing society, industry and science.

The Paul Scherrer Institute (PSI) is a multi-disciplinary research institute for natural and engineering sciences in Switzerland. It is located in the Canton of Aargau in the municipalities Villigen and Würenlingen on either side of the River Aare, and covers an area over 35 hectares in size. Like Swiss Federal Institute of Technology ETH Zürich [Eidgenössische Technische Hochschule Zürich)](CH) and EPFL (Swiss Federal Institute of Technology in Lausanne) [École polytechnique fédérale de Lausanne](CH), PSI belongs to the Domain of the Swiss Federal Institutes of Technology (ETH Domain) [ETH-Bereich; Domaine des Écoles polytechniques fédérales](CH). The PSI employs around 2100 people. It conducts basic and applied research in the fields of matter and materials, human health, and energy and the environment. About 37% of PSI’s research activities focus on material sciences, 24% on life sciences, 19% on general energy, 11% on nuclear energy and safety, and 9% on particle physics.

PSI develops, builds and operates large and complex research facilities and makes them available to the national and international scientific communities. In 2017, for example, more than 2500 researchers from 60 different countries came to PSI to take advantage of the concentration of large-scale research facilities in the same location, which is unique worldwide. About 1900 experiments are conducted each year at the approximately 40 measuring stations in these facilities.

In recent years, the institute has been one of the largest recipients of money from the Swiss lottery fund.

Research and specialist areas

PSI develops, builds and operates several accelerator facilities, e. g. a 590 MeV high-current cyclotron, which in normal operation supplies a beam current of about 2.2 mA. PSI also operates four large-scale research facilities: a synchrotron light source (SLS), which is particularly brilliant and stable, a spallation neutron source (SINQ), a muon source (SμS) and an X-ray free-electron laser (SwissFEL). This makes PSI currently (2020) the only institute in the world to provide the four most important probes for researching the structure and dynamics of condensed matter (neutrons, muons and synchrotron radiation) on a campus for the international user community. In addition, HIPA’s target facilities also produce pions that feed the muon source and the Ultracold Neutron source UCN produces very slow, ultracold neutrons. All these particle types are used for research in particle physics.

One of ten national laboratories overseen and primarily funded by the DOE(US) Office of Science, DOE’s Brookhaven National Laboratory (US) conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world. Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University(US), the largest academic user of Laboratory facilities, and Battelle(US), a nonprofit, applied science and technology organization.

Research at BNL specializes in nuclear and high energy physics, energy science and technology, environmental and bioscience, nanoscience and national security. The 5,300 acre campus contains several large research facilities, including the Relativistic Heavy Ion Collider [below] and National Synchrotron Light Source II [below]. Seven Nobel prizes have been awarded for work conducted at Brookhaven lab.

BNL is staffed by approximately 2,750 scientists, engineers, technicians, and support personnel, and hosts 4,000 guest investigators every year. The laboratory has its own police station, fire department, and ZIP code (11973). In total, the lab spans a 5,265-acre (21 km^2) area that is mostly coterminous with the hamlet of Upton, New York. BNL is served by a rail spur operated as-needed by the New York and Atlantic Railway. Co-located with the laboratory is the Upton, New York, forecast office of the National Weather Service.

Major programs

Although originally conceived as a nuclear research facility, Brookhaven Lab’s mission has greatly expanded. Its foci are now:

Nuclear and high-energy physics
Physics and chemistry of materials
Environmental and climate research
Energy research
Structural biology
Accelerator physics


Brookhaven National Lab was originally owned by the Atomic Energy Commission(US) and is now owned by that agency’s successor, the United States Department of Energy (DOE). DOE subcontracts the research and operation to universities and research organizations. It is currently operated by Brookhaven Science Associates LLC, which is an equal partnership of Stony Brook University(US) and Battelle Memorial Institute(US). From 1947 to 1998, it was operated by Associated Universities, Inc. (AUI) (US), but AUI lost its contract in the wake of two incidents: a 1994 fire at the facility’s high-beam flux reactor that exposed several workers to radiation and reports in 1997 of a tritium leak into the groundwater of the Long Island Central Pine Barrens on which the facility sits.


Following World War II, the US Atomic Energy Commission was created to support government-sponsored peacetime research on atomic energy. The effort to build a nuclear reactor in the American northeast was fostered largely by physicists Isidor Isaac Rabi and Norman Foster Ramsey Jr., who during the war witnessed many of their colleagues at Columbia University leave for new remote research sites following the departure of the Manhattan Project from its campus. Their effort to house this reactor near New York City was rivalled by a similar effort at the Massachusetts Institute of Technology (US) to have a facility near Boston, Massachusettes(US). Involvement was quickly solicited from representatives of northeastern universities to the south and west of New York City such that this city would be at their geographic center. In March 1946 a nonprofit corporation was established that consisted of representatives from nine major research universities — Columbia University(US), Cornell University(US), Harvard University(US), Johns Hopkins University(US), Massachusetts Institute of Technology(US), Princeton University(US), University of Pennsylvania(US), University of Rochester(US), and Yale University(US).

Out of 17 considered sites in the Boston-Washington corridor, Camp Upton on Long Island was eventually chosen as the most suitable in consideration of space, transportation, and availability. The camp had been a training center from the US Army during both World War I and World War II. After the latter war, Camp Upton was deemed no longer necessary and became available for reuse. A plan was conceived to convert the military camp into a research facility.

On March 21, 1947, the Camp Upton site was officially transferred from the U.S. War Department to the new U.S. Atomic Energy Commission (AEC), predecessor to the U.S. Department of Energy (DOE).

Research and facilities

Reactor history

In 1947 construction began on the first nuclear reactor at Brookhaven, the Brookhaven Graphite Research Reactor. This reactor, which opened in 1950, was the first reactor to be constructed in the United States after World War II. The High Flux Beam Reactor operated from 1965 to 1999. In 1959 Brookhaven built the first US reactor specifically tailored to medical research, the Brookhaven Medical Research Reactor, which operated until 2000.

Accelerator history

In 1952 Brookhaven began using its first particle accelerator, the Cosmotron. At the time the Cosmotron was the world’s highest energy accelerator, being the first to impart more than 1 GeV of energy to a particle.

The Cosmotron was retired in 1966, after it was superseded in 1960 by the new Alternating Gradient Synchrotron (AGS).

The AGS was used in research that resulted in 3 Nobel prizes, including the discovery of the muon neutrino, the charm quark, and CP violation.

In 1970 in BNL started the ISABELLE project to develop and build two proton intersecting storage rings.

The groundbreaking for the project was in October 1978. In 1981, with the tunnel for the accelerator already excavated, problems with the superconducting magnets needed for the ISABELLE accelerator brought the project to a halt, and the project was eventually cancelled in 1983.

The National Synchrotron Light Source (US) operated from 1982 to 2014 and was involved with two Nobel Prize-winning discoveries. It has since been replaced by the National Synchrotron Light Source II (US) [below].

After ISABELLE’S cancellation, physicist at BNL proposed that the excavated tunnel and parts of the magnet assembly be used in another accelerator. In 1984 the first proposal for the accelerator now known as the Relativistic Heavy Ion Collider (RHIC)[below] was put forward. The construction got funded in 1991 and RHIC has been operational since 2000. One of the world’s only two operating heavy-ion colliders, RHIC is as of 2010 the second-highest-energy collider after the Large Hadron Collider(CH). RHIC is housed in a tunnel 2.4 miles (3.9 km) long and is visible from space.

On January 9, 2020, It was announced by Paul Dabbar, undersecretary of the US Department of Energy Office of Science, that the BNL eRHIC design has been selected over the conceptual design put forward by DOE’s Thomas Jefferson National Accelerator Facility [Jlab] (US) as the future Electron–ion collider (EIC) in the United States.

In addition to the site selection, it was announced that the BNL EIC had acquired CD-0 (mission need) from the Department of Energy. BNL’s eRHIC design proposes upgrading the existing Relativistic Heavy Ion Collider, which collides beams light to heavy ions including polarized protons, with a polarized electron facility, to be housed in the same tunnel.

Other discoveries

In 1958, Brookhaven scientists created one of the world’s first video games, Tennis for Two. In 1968 Brookhaven scientists patented Maglev, a transportation technology that utilizes magnetic levitation.

Major facilities

Relativistic Heavy Ion Collider (RHIC), which was designed to research quark–gluon plasma and the sources of proton spin. Until 2009 it was the world’s most powerful heavy ion collider. It is the only collider of spin-polarized protons.
Center for Functional Nanomaterials (CFN), used for the study of nanoscale materials.
BNL National Synchrotron Light Source II(US), Brookhaven’s newest user facility, opened in 2015 to replace the National Synchrotron Light Source (NSLS), which had operated for 30 years.[19] NSLS was involved in the work that won the 2003 and 2009 Nobel Prize in Chemistry.
Alternating Gradient Synchrotron, a particle accelerator that was used in three of the lab’s Nobel prizes.
Accelerator Test Facility, generates, accelerates and monitors particle beams.
Tandem Van de Graaff, once the world’s largest electrostatic accelerator.
Computational Science resources, including access to a massively parallel Blue Gene series supercomputer that is among the fastest in the world for scientific research, run jointly by Brookhaven National Laboratory and Stony Brook University.
Interdisciplinary Science Building, with unique laboratories for studying high-temperature superconductors and other materials important for addressing energy challenges.
NASA Space Radiation Laboratory, where scientists use beams of ions to simulate cosmic rays and assess the risks of space radiation to human space travelers and equipment.

Off-site contributions

It is a contributing partner to ATLAS experiment, one of the four detectors located at the Large Hadron Collider (LHC).

It is currently operating at CERN near Geneva, Switzerland.

Brookhaven was also responsible for the design of the SNS accumulator ring in partnership with Spallation Neutron Source at DOE’s Oak Ridge National Laboratory (US), Tennessee.

Brookhaven plays a role in a range of neutrino research projects around the world, including the Daya Bay Neutrino Experiment (CN) nuclear power plant, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China.