Periodic Reporting for period 3 - HERO (Hidden, entangled and resonating orders)
Berichtszeitraum: 2022-05-01 bis 2023-10-31
The HERO (Hidden, Entangled and Resonating Orders) project aims to identify novel kinds of hidden orders in materials, both to expose new fundamental physics and to engineer new properties with potential technological applications. In condensed matter systems, the atoms, electrons or spins sometimes arrange themselves in ways that result in unexpected properties that cannot be detected by conventional experimental probes. A historical example is the case of apparently non-magnetic manganese oxide (MnO) in which an unexpected experimental observation was made almost one hundred years ago: A cusp in the specific heat as a function of temperature, indicative of a phase transition, was found to coincide with a cusp in the magnetic susceptibility, suggesting that the phase transition had a magnetic origin. An explanation was proposed by Neel, who showed theoretically that the observed behaviour was consistent with hidden antiferromagnetic order, but verification required the development of a new characterization technique (neutron diffraction), which was able to directly measure the antiparallel alignment of neighbouring Mn magnetic moments.
In his Nobel Prize lecture, Neel made the statement that “while a large number of antiferromagnetic materials are now known, they are extremely interesting from the theoretical viewpoint, but do not seem to have any applications." Today, antiferromagnetic materials underpin multi-billion dollar industries as the exchange bias component in magnetic sensors and are promising for possible future spintronic devices.
In the HERO project, we are searching systematically for new forms of hidden order in three previously unexplored directions.
First, composite orders that are derived from correlations between conventional classical order parameters. Here, an example is motivated by multiferroic materials, which are simultaneously ferromagnetic and ferroelectric. That is, the order parameters describing the magnetic moment and
the electric dipole are non-zero. Instead of this revealed composite order, we are identifying scenarios in which both magnetic and electric dipole orders are zero but their product is not. Such materials have no net magnetization or electric polarization, but exhibit complex magnetic and dielectric susceptibilities associated with the hidden coupled magneto-electric order.
Second, entangled orders between quantum variables, such as at quantum multicritical points where different forms of order simultaneously appear near zero temperature.
Third, orders arising from dynamical effects such as quantum fluctuations or from external ac drive fields.
In his Nobel Prize lecture, Neel made the statement that “while a large number of antiferromagnetic materials are now known, they are extremely interesting from the theoretical viewpoint, but do not seem to have any applications." Today, antiferromagnetic materials underpin multi-billion dollar industries as the exchange bias component in magnetic sensors and are promising for possible future spintronic devices.
In the HERO project, we are searching systematically for new forms of hidden order in three previously unexplored directions.
First, composite orders that are derived from correlations between conventional classical order parameters. Here, an example is motivated by multiferroic materials, which are simultaneously ferromagnetic and ferroelectric. That is, the order parameters describing the magnetic moment and
the electric dipole are non-zero. Instead of this revealed composite order, we are identifying scenarios in which both magnetic and electric dipole orders are zero but their product is not. Such materials have no net magnetization or electric polarization, but exhibit complex magnetic and dielectric susceptibilities associated with the hidden coupled magneto-electric order.
Second, entangled orders between quantum variables, such as at quantum multicritical points where different forms of order simultaneously appear near zero temperature.
Third, orders arising from dynamical effects such as quantum fluctuations or from external ac drive fields.
The melting and condensation of water has been a continuous source of fascination, and it has long been recognized that at the root of these phenomena are a complex interplay of quantum and thermal effects. An interesting challenge is to find systems where the role of thermal fluctuations is suppressed. In the project period to date, we have reported on the discovery, in a joint experimental and theoretical study, that critical point in Sr2Cu(BO3)2 is a quantum magnetic analogue to the critical point in water terminating the first order transition line between the vapor and liquid phases.
Another exciting result is the development of a theory of dynamically induced magnetism in potassium tantalate (KTaO3, otherwise designated KTO). In this approach, fluctuating electric dipoles induce magnetization. Hence, a material with paraelectric fluctuations can develop magnetic signatures if dynamically driven. We identified the paraelectric KTaO 3 (KTO) as a prime candidate for the observation of this phenomenon. We show that when a KTO sample is exposed to a circularly polarized laser pulse, the dynamically-induced ionic magnetic moments are of the order of 5% of the nuclear magneton per unit cell, in agreement with recent experiments.
The principal event outside the project has been the COVID-19 pandemic. This has not only interrupted scientific work dedicated to the main goals of HERO, but has also presented problems, which can be dealt with using some of the basic methods of statistical physics and dynamical systems that also underpin HERO. HERO has accordingly also made contributions in this direction, most notably in proposing a feedback and control model for regulating the pandemic so as to maintain the demands on medical services at acceptable levels.
Another exciting result is the development of a theory of dynamically induced magnetism in potassium tantalate (KTaO3, otherwise designated KTO). In this approach, fluctuating electric dipoles induce magnetization. Hence, a material with paraelectric fluctuations can develop magnetic signatures if dynamically driven. We identified the paraelectric KTaO 3 (KTO) as a prime candidate for the observation of this phenomenon. We show that when a KTO sample is exposed to a circularly polarized laser pulse, the dynamically-induced ionic magnetic moments are of the order of 5% of the nuclear magneton per unit cell, in agreement with recent experiments.
The principal event outside the project has been the COVID-19 pandemic. This has not only interrupted scientific work dedicated to the main goals of HERO, but has also presented problems, which can be dealt with using some of the basic methods of statistical physics and dynamical systems that also underpin HERO. HERO has accordingly also made contributions in this direction, most notably in proposing a feedback and control model for regulating the pandemic so as to maintain the demands on medical services at acceptable levels.
The HERO project has an integrated experimental and theoretical approach to the problems outlined above and requires the development of novel methods on both fronts. During this initial period, we have initiated substantial subprojects to provide new capabilities to visualize hidden order, among them a laser slicing upgrade of the Swiss Free Electron Laser (SwissFEL) to produce subfemtosecond pulses, i.e. pulses, which are shorter than the time it takes for electrons to travel between atoms in many solids.
Notwithstanding the COVID-19 pandemic, we have made progress on the complex installation where strongly relativistic electrons interact with a high power pulsed laser beam. Other progress beyond the state of the art on the experimental technology front has been the development of a novel technique exploiting optical communications components and software control (via the same paradigm as for data routing on the internet) for ultra-high resolution THz spectroscopy. The THz domain is the region between microwave and far infrared domains, and entails oscillations with periods of order femto- to picoseconds, corresponding to energies of order room temperature and, therefore, also to the formation energies of many interesting phases of matter, including of course those hosting hidden order.
One of the big challenges for our understanding of the universe concerns the nature of dark matter, which by definition is a difficult topic for experimental observation. We have found that the rate at which atoms can be ionized via dark matter-electron scattering can in general be expressed in terms of four independent atomic responses. We then computed the response of atomic argon and xenon targets, which are used in operating dark matter search experiments We used our results to set 90% confidence level exclusion limits on the strength of a wide range of dark matter-electron interactions from the null result of these experiments. The novel response functions that we discovered encode properties of electrons that do not interact with conventional experimental probes, suggesting the use of the dark matter wind as a probe to reveal new kinds of hidden electronic order in materials.
The results expected at the end of the project will be new forms of “Hidden” Order , derived either from correlations between classical order parameters, which could even be vanishing due to quantum fluctuations or from external ac drive fields. Quantum multicritical points where different forms of order simultaneously appear near zero temperature will be considered with special attention to the effects of Entanglement between mesoscopic quantum variables associated with the multiple orders.
Finally, we will learn the consequences of resonant level crossings for symmetry-restoring modes associated with different orders.
Notwithstanding the COVID-19 pandemic, we have made progress on the complex installation where strongly relativistic electrons interact with a high power pulsed laser beam. Other progress beyond the state of the art on the experimental technology front has been the development of a novel technique exploiting optical communications components and software control (via the same paradigm as for data routing on the internet) for ultra-high resolution THz spectroscopy. The THz domain is the region between microwave and far infrared domains, and entails oscillations with periods of order femto- to picoseconds, corresponding to energies of order room temperature and, therefore, also to the formation energies of many interesting phases of matter, including of course those hosting hidden order.
One of the big challenges for our understanding of the universe concerns the nature of dark matter, which by definition is a difficult topic for experimental observation. We have found that the rate at which atoms can be ionized via dark matter-electron scattering can in general be expressed in terms of four independent atomic responses. We then computed the response of atomic argon and xenon targets, which are used in operating dark matter search experiments We used our results to set 90% confidence level exclusion limits on the strength of a wide range of dark matter-electron interactions from the null result of these experiments. The novel response functions that we discovered encode properties of electrons that do not interact with conventional experimental probes, suggesting the use of the dark matter wind as a probe to reveal new kinds of hidden electronic order in materials.
The results expected at the end of the project will be new forms of “Hidden” Order , derived either from correlations between classical order parameters, which could even be vanishing due to quantum fluctuations or from external ac drive fields. Quantum multicritical points where different forms of order simultaneously appear near zero temperature will be considered with special attention to the effects of Entanglement between mesoscopic quantum variables associated with the multiple orders.
Finally, we will learn the consequences of resonant level crossings for symmetry-restoring modes associated with different orders.