Quantum simulation: A better understanding of magnetism

Quantum simulation: A better understanding of magnetism
(More at link: https://www.sciencedaily.com/releases/2015/11/151120104047.htm)

Date: November 20, 2015
Source: Heidelberg, Universität
Summary: Physicists have used ultracold atoms to imitate the behavior of electrons in a solid. Researchers have devised a new way to study the phenomenon of magnetism. Using ultracold atoms at near absolute zero, they prepared a model that simulates the behavior of electrons in a solid, which enables the investigation of magnetic properties.
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Atoms (shown in green and blue) are held in a trap of laser light (red) in which they can move in one dimension only. The atoms can point either up (green) or down (blue), similar to a needle in a compass. When the atoms do not interact, they can move freely in the trap (top picture); they have no discernible order. When repulsive interactions between the atoms are strong (bottom picture), they arrange themselves in the trap, with each atom pointing in the opposite direction of its neighbour.
Credit: Heidelberg University

Researchers at Heidelberg University have devised a new way to study the phenomenon of magnetism. Using ultracold atoms at near absolute zero, they prepared a model that simulates the behaviour of electrons in a solid, which enables the investigation of magnetic properties. The findings of the team led by Prof. Selim Jochim of the Institute for Physics are expected to contribute to a better understanding of the fundamental processes in solids and lead to the development of new types of materials over the long term. The results of their quantum simulation research, conducted with physicists from Hannover and Lund (Sweden), appeared in the journal Physical Review Letters.

Magnetism has been known for over 2,000 years, and was used early on to develop the compass, whose needles align themselves with the earth's magnetic field. Nonetheless, the microscopic causes of magnetism were not understood until the development of quantum mechanics at the beginning of the 20th century. One of the most important discoveries was that electrons in a solid behave like tiny compass needles that align themselves with an external magnetic field and also affect each other. The magnetic properties of a solid depend on how adjacent electrons arrange themselves relative to one another. For instance in ferromagnetic substances such as iron, all electrons point in the same direction. In antiferromagnetism, however, each electron points in the opposite direction of its neighbour.

The Heidelberg physicists used very few atoms, namely four, for their quantum simulation. "Precisely preparing such a small number of atoms is a major technical undertaking. It allows us, however, to control the state of the atoms with extreme precision," explains Simon Murmann, Prof. Jochim's doctoral student in charge of the experiments who has just completed his thesis on the subject. The atoms are held in a laser light trap that allows movement in only one dimension. They are subject to virtually the same physical laws as electrons in a solid, but the physicists are able to precisely control the interactions of the atoms. "Initially, there is no interaction between the atoms. In this state, they can move freely inside the trap without any fixed arrangement. But when we introduce increasing repulsion between the atoms, they can no longer pass one another and end up forming a chain. Each atom in the chain points in the opposite direction of its neighbour, one up and one down. This brings about an antiferromagnetic state," explains the Heidelberg scientist.
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Journal Reference:

S. Murmann, F. Deuretzbacher, G. Zürn, J. Bjerlin, S. M. Reimann, L. Santos, T. Lompe, S. Jochim. Antiferromagnetic Heisenberg Spin Chain of a Few Cold Atoms in a One-Dimensional Trap. Physical Review Letters, 2015; 115 (21) DOI: 10.1103/PhysRevLett.115.215301


https://www.sciencedaily.com/releases/2015/11/151120104047.htm

Correlated magnets made out of single atoms

Date: September 29, 2016
Source: Max Planck Institute of Quantum Optics
Summary: Scientists have observed antiferromagnetic correlations in one-dimensional fermionic quantum many-body systems, outlines a new report.

In (a) an originally obtained picture of a one-dimensional atomic chain is shown. The thick horizontal lines illustrate the barrier between different chains. In each chain, an atom appearing on the upper side of the thin dashed horizontal line has upward pointing magnetic moment (red) and vice versa as shown in the reconstructed image (b). In some cases, doubly occupied sites or holes (empty sites) are detected.
Credit: Martin Boll, Quantum Many-Body Systems Division, MPQ

Solid state physics offers a rich variety of intriguing phenomena, several of which are not yet fully understood. Experiments with fermionic atoms in optical lattices get very close to imitating the behaviour of electrons in solid state crystals, thus forming a well-controlled quantum simulator for these systems. Now a team of scientists around Professor Immanuel Bloch and Dr. Christian Groß at the Max Planck Institute of Quantum Optics have observed the emergence of antiferromagnetic order over a correlation length of several lattice sites in a chain of fermionic atoms. Contrary to the ferromagnetism we experience in everyday life, these antiferromagnets are characterized by an alternating alignment of the elementary magnetic moment associated with each electron or atom. Combining their quantum gas microscope with advanced local manipulation techniques, the scientists were able to simultaneously observe the spin and the density distribution with single-site resolution and single atom sensitivity. By approaching the conditions prevailing in macroscopic crystals with fermionic quantum many-body systems, one hopes to achieve a better understanding of phenomena such as the so-called high-temperature superconductivity.

The experiment started with cooling a cloud of fermionic lithium-6 atoms down to extremely low temperatures, a millionth of a Kelvin above absolute zero. These ultracold fermions were then trapped by light fields and forced into a single plane, which in turn was further split in several one-dimensional tubes. Finally, an optical lattice was applied along the tubes mimicking the periodic potential that electrons see in a real material.

On average, the one-dimensional optical lattices were completely filled, meaning that each lattice site was occupied with exactly one atom. Two internal quantum states of the lithium atoms mimic the magnetic moment of the electrons, which can point either upwards or downwards. As long as the temperature of the system is high compared to the magnetic interaction between these spins, only the density distribution of the system shows a regular pattern dictated by the optical lattice. However, below a certain temperature the magnetic moments of neighbouring atoms are expected to anti-align, leading to antiferromagenic correlations. "These correlations arise because the system aims to lower its energy," Martin Boll, doctoral student at the experiment, explains. "The underlying mechanism is called "superexchange" which means that the magnetic moments of neighbouring atoms exchange their directions."

The team around Christian Groß and Immanuel Bloch had to tackle two main challenges: First, it was necessary to measure the particle density with high resolution to unambiguously identify single particles and holes on their individual lattice sites. This was achieved with the quantum gas microscope where a high resolution objective images the atoms all at once, such that a series of photographic snapshots of the atomic gas can be taken. "The second really big challenge was the separation of atoms based on their magnetic orientations," says Martin Boll. "To this end, we combined an optical superlattice with a magnetic gradient that shifted the potential minima depending on the orientation of the magnetic moment. As a consequence, opposite magnetic moments were separated into two different sites of the local double well potential created by the superlattice. In a series of measurements we have tuned this method to such a degree that we obtained a splitting fidelity of nearly 100 percent."

Having all these tools at hand, the team succeeded to observe the emergence of antiferromagnetic correlations that extended over three sites, well beyond nearest-neighbours. "Quantum simulations with fermions in optical lattices is of particular interest because it may lead to a better understanding of the so-called "high-temperature" superconductivity for which the interplay of holes and antiferromagnetic correlations is believed to be crucial.," Dr. Christian Groß points out. "In the near future, we might be able to even prepare our samples with a certain degree of hole-doping that resembles the conditions in superconducting materials."

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Materials provided by Max Planck Institute of Quantum Optics. Note: Content may be edited for style and length.

Journal Reference:

M. Boll, T. A. Hilker, G. Salomon, A. Omran, J. Nespolo, L. Pollet, I. Bloch, C. Gross. Spin- and density-resolved microscopy of antiferromagnetic correlations in Fermi-Hubbard chains. Science, 2016; 353 (6305): 1257 DOI: 10.1126/science.aag1635

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