Computers Math

In topological materials, electrons can display behaviour that is fundamentally different from that in ‘conventional’ matter, and the magnitude of many such ‘exotic’ phenomena is directly proportional to an entity known as the Chern number. New experiments establish for the first time that the theoretically predicted maximum Chern number can be reached — and controlled — in a real material.
When the Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics 2016 to David Thouless, Duncan Haldane and Michael Kosterlitz, they lauded the trio for having “opened the door on an unknown world where matter can assume strange states.” Far from being an oddity, the discoveries of topological phase transitions and topological phases of matter, to which the three theoreticians have contributed so crucially, has grown into one of the most active fields of research in condensed matter physics today. Topological materials hold the promise, for instance, to lead to novel types of electronic components and superconductors, and they harbour deep connections across areas of physics and mathematics. While new phenomena are discovered routinely, there are fundamental aspects yet to be settled. One of those is just how ‘strong’ topological phenomena can be in a real material. Addressing that question, an international team of researchers led by PSI postdoctoral researcher Niels Schröter provide now an important benchmark. Writing in Science, they report experiments in which they observed that in the topological semimetal palladium gallium (PdGa) one of the most common classifiers of topological phenomena, the Chern number, can reach the maximum value that is allowed in any metallic crystal. That this is possible in a real material has never been shown before. Moreover, the team has established ways to control the sign of the Chern number, which might bring new opportunities for exploring, and exploiting, topological phenomena.
Developed to the maximum
In theoretical works it had been predicted that in topological semimetals the Chern number cannot exceed a magnitude of four. As candidate systems displaying phenomena with such maximal Chern numbers, chiral crystals were proposed. These are materials whose lattice structures have a well-defined handedness, in the sense that they cannot transformed into their mirror image by any combination of rotations and translations. Several candidate structures have been studied. A conclusive experimental observation of a Chern number of plus or minus four, however, remained elusive. The previous efforts have been hindered by two factors in particular. First, a prerequisite for realizing a maximal Chern number is the presence of spin-orbit coupling, and at least in some of the materials studied so far, that coupling is relatively low, making it difficult to resolve the splittings of interest. Second, preparing clean and flat surfaces of relevant crystals has been highly challenging, and as a consequence spectroscopic signatures tended to be washed out.
Schröter et al. have overcome both of these limitations by working with PdGa crystals. The material displays strong spin-orbit coupling, and well-established methods exist for producing immaculate surfaces. In addition, at the Advanced Resonant Spectroscopies (ADRESS) beamline of the Swiss Light Source at PSI, they had unique capabilities at their disposal for high-resolution ARPES experiments and thus to resolve the predicted tell-tale spectroscopic patterns. In combination with further measurements at the Diamond Light Source (UK) and with dedicated ab initio calculations, these data revealed hard and fast signatures in the electronic structure of PdGa that left no doubt that the maximal Chern number has been realized.
A hand on the Chern number
The team went one step further, beyond the observation of a maximal Chern number. They showed that the chiral nature of the PdGa crystals offers a possibility to control the sign of that number as well. To demonstrate such control, they grew samples that were either leftor right-handed (see the figure). When they looked then at the electronic structures of the two enantiomers, they found that the chirality of the crystals is reflected in the chirality of the electronic wave function. Taken together, this means that in chiral semimetals the handedness, which can be determined during crystal growth, can used to control topological phenomena emerging from the behaviour of the electrons in the material. This sort of control opens a trove of new experiments. For example, novel effects can be expected to arise at the interface between different enantiomers, one with Chern number +4 and the other one with -4. And there are real prospects for applications, too. Chiral topological semimetals can host fascinating phenomena such as quantized photocurrents. Intriguingly, PdGa is known for its catalytic properties, inviting the question about the role of topological phenomena in such processes.
Finally, the findings now obtained for PdGa emerge from electronic band properties that are shared by many other chiral compounds — meaning that the corner of the “unknown world where matter can assume strange states” into which Schröter and colleagues have now ventured is likely to have a lot more to offer.

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Researchers at the University of Tsukuba used advanced metrics from information theory to describe the collective behavior of small schools of ayu fish. They found that the overall dynamics were noticeably different for groups of three or more, compared with smaller groups, even over very short timescales. This work may help shed light on fundamental problems in complexity theory and assist in the development of cooperative biomimetic swarming robots.
Some of the most difficult questions in science today deal with the same fundamental question: How can complex dynamics arise from simple, local interactions? For example, schools of fish and flocks of birds can move and turn in near perfect synchrony without any top-down control from a leader. To try to make progress on this question, integrated information theory (IIT) has been used to provide a mathematical framework for measuring how information passes back and forth from individuals to the group as a whole.
A team including Professor Takayuki Niizato at the University of Tsukuba studied the swimming of Ayu, which are small fish related to smelts. Ayu were randomly chosen and placed in a tank in groups of 2, 3, 4, or 5 fish.
Explains Professor Niizato, “The trajectory of each fish was tracked, and a computer calculated three binary parameters for every fish at each time step. These parameters were as follows: if the subject fish was close to another fish, if the fish was turning, and if another fish was in its field of view. We found that, over times ranging from 40 milliseconds to one second, a grouping of three fish acted very differently than a pair.”
In this study, “integrated information” quantified the extent to which the cause and effect in a system can be explained by the repertoires of its components. And “integrated conceptual information” was used as an expression of the fish school’s group collective action, roughly how much the behavior of individual members is dependent on the behavior of the group.
“The aim of IIT is to try to shift the paradigm from ‘what a system does’ to ‘what a system is.’ The former tries to analyze the system on the basis of observable behavior, while the latter tries to determine its intrinsic causal structure,” says Professor Niizato.
This work may help make inroads in some truly difficult questions surrounding group dynamics that emerge naturally when simple components join to form a complex group. This may also aid in the development of “swarming” robots that, like schools of fish or ant colonies, make use of this principle to achieve complicated goals.

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COVID-19 has placed tremendous pressure on health care systems, not only for critical care but also from an anxious public looking for answers.
Research from the Indiana University Kelley School of Business found that chatbots — software applications that conduct online chats via text or text-to-speech — working for reputable organizations can ease the burden on medical providers and offer trusted guidance to those with symptoms.
Researchers conducted an online experiment with 371 participants who viewed a COVID-19 screening session between a hotline agent — chatbot or human — and a user with mild or severe symptoms.
They studied whether chatbots were seen as being persuasive, providing satisfying information that likely would be followed. Their results showed a slight negative bias against chatbots’ ability, perhaps due to recent press reports. When the perceived ability is the same, however, participants reported that they viewed chatbots more positively than human agents, which is good news for health care organizations struggling to meet user demand for screening services.
“The primary factor driving user response to screening hotlines — human or chatbot — is perceptions of the agent’s ability,” said Alan Dennis, the John T. Chambers Chair of Internet Systems at Kelley and corresponding author of the paper, “User reactions to COVID-19 screening chatbots from reputable providers.” “When ability is the same, users view chatbots no differently or more positively than human agents.”
Other authors on the paper, forthcoming in the Journal of the American Medical Informatics Association, are Antino Kim, assistant professor of operations and decision technologies at Kelley; and Sezgin Ayabakan, assistant professor of management information systems, and doctoral candidate Mohammad Rahimi, both at Temple University’s Fox School of Business.
Even before the pandemic, chatbots were identified as a technology that could speed up how people interact with researchers and find medical information online.
“Chatbots are scalable, so they can meet an unexpected surge in demand when there is a shortage of qualified human agents,” Dennis, Kim and their co-authors wrote, adding that chatbots “can provide round-the-clock service at a low operational cost.
“This positive response may be because users feel more comfortable disclosing information to a chatbot, especially socially undesirable information, because a chatbot makes no judgment,” researchers wrote. “The CDC, the World Health Organization, UNICEF and other health organizations caution that the COVID-19 outbreak has provoked social stigma and discriminatory behaviors against people of certain ethnic backgrounds, as well as those perceived to have been in contact with the virus. This is truly an unfortunate situation, and perhaps chatbots can assist those who are hesitant to seek help because of the stigma.”
The primary factor driving perceptions of ability was the user’s trust in the provider of the screening hotline.
“Proactively informing users of the chatbot’s ability is important,” the authors wrote. “Users need to understand that chatbots use the same up-to-date knowledge base and follow the same set of screening protocols as human agents. … Because trust in the provider strongly influences perceptions of ability, building on the organization’s reputation may also prove useful.”

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The quantum spin liquid (QSL) state is an exotic state of matter where the spin of electrons, which generally exhibits order at low temperatures, remains disordered. Now, scientists have developed a new material where a two-dimensional QSL state can be experimentally observed, advancing our knowledge of spin behavior, and getting us closer to next-generation ”spintronic” devices. More

Cognitive behavioural therapy delivered electronically to treat people with depression is more effective than face to face, suggests an evidence review led by McMaster University.
Based on randomized control trials, the systematic review and analysis revealed that cognitive behavioural therapy that connected therapists and patients through such modes as web-based applications, video-conferencing, email and texting, improved patients’ symptoms better than face to face when measured using standardized mood symptoms scales. As well, there was no difference in the level of satisfaction or function between the two methods of delivery.
The details were published in EClinicalMedicine, published by The Lancet.
“Although this study started before the current COVID-19 pandemic, it is timely and assuring that treatment delivered electronically works as well if not better than face to face and there is no compromise on the quality of care that patients are receiving during this stressful time,” said corresponding author Zena Samaan, associate professor of psychiatry and behavioural neurosciences at McMaster and a psychiatrist at St. Joseph’s Healthcare Hamilton.
Cognitive behavioural therapy is a type of psychotherapy widely used to treat depression. However, limited resource availability poses several barriers to patients seeking access to care, including lengthy wait times and geographical limitations.
In this evidence review, researchers identified 17 randomized control trials comparing therapist-supported cognitive behavioural therapy delivered electronically to face to face cognitive behavioural therapy. The studies were conducted between 2003 and 2018 in the United States, Australia, Netherlands, Switzerland, Sweden and the United Kingdom.
Samaan said the findings of the meta-analysis debunk widely-held perceptions about psychotherapy.
“The common understanding was that face to face psychotherapy has the advantage of the connection with the therapist and this connection is in part what makes the difference in treatment,” she said.
“However, it is not surprising that electronic interventions are helpful in that they offer flexibility, privacy and no travel time, time off work, transport or parking costs. It makes sense that people access care, especially mental health care, when they need it from their own comfort space.”
Samaan noted that the findings support advocacy and widespread implementation of electronic cognitive behavioural therapy.
“Electronic options should be considered to be implemented for delivering therapy to patients,” she said. “This can potentially vastly improve access for patients, especially those in rural or underserved areas, and during pandemics.”
This work did not receive any external funding.

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The Large Hadron Collider (LHC) near Geneva, Switzerland became famous around the world in 2012 with the detection of the Higgs boson. The observation marked a crucial confirmation of the Standard Model of particle physics, which organizes the subatomic particles into groups similar to elements in the periodic table from chemistry. The U.S. Department of […] More

Parents may worry that spending time on their smartphones has a negative impact on their relationships with their children. However, a new comprehensive analysis published in the Journal of Child Psychology and Psychiatry found that this is unlikely to be the case. In the analysis of data from 3, 659 parent-based surveys, the authors tested […] More

A team of physicists and chemists has produced the first porous graphene ribbons in which specific carbon atoms in the crystal lattice are replaced with nitrogen atoms. These ribbons have semiconducting properties that make them attractive for applications in electronics and quantum computing. More