Michael Tobar - Academia.edu (original) (raw)
Papers by Michael Tobar
arXiv (Cornell University), Oct 18, 2017
Sensors
The creation of electromagnetic energy may be realised by engineering a device with a method of t... more The creation of electromagnetic energy may be realised by engineering a device with a method of transduction, which allows an external energy source, such as mechanical, chemical, nuclear, etc., to be impressed into the electromagnetic system through a mechanism that enables the separation of opposite polarity charges. For example, a voltage generator, such as a triboelectric nanogenerator, enables the separation of charges through the transduction of mechanical energy, creating an active physical dipole in the static case, or an active Hertzian dipole in the time-dependent case. The net result is the creation of a static or time-dependent permanent polarisation, respectively, without an applied electric field and with a non-zero vector curl. This system is the dual of a magnetic solenoid or permanent magnet excited by a circulating electrical current or fictitious bound current, respectively, which supplies a magnetomotive force described by a magnetic vector potential and a magnet...
The conversion of external energy into electricity is the foundation of power station and energy ... more The conversion of external energy into electricity is the foundation of power station and energy harvesting operation. The external source supplies an impressed force per unit charge to free or bound charge to produces AC electricity. We analyze the electrodynamics of ideal electricity generators through a time dependent permanent polarization without any applied electric field, which modifies the constitutive relations and is essential to oscillate charge in a lossless way. For both cases, we show that Maxwell's equations, and in particular Faraday's law are modified, along with the required boundary conditions through the addition of an effective impressed magnetic current boundary source. For the free charge case, we highlight the example of an electromagnetic generator based on Lorentz force, where the impressed force per unit charge that polarizes the conductor comes from mechanical motion of free charge with an impressed velocity of a conductor relative to a stationary...
![Research paper thumbnail of Erratum for "Axion Dark Matter Coupling to Resonant Photons via Magnetic Field" [arXiv:1512.05547]](https://attachments.academia-assets.com/105282742/thumbnails/1.jpg)
](A recent comment highlights a discussion at the PATRAS Workshop on Axions, WIMPs and WISPs relati... more A recent comment highlights a discussion at the PATRAS Workshop on Axions, WIMPs and WISPs relating to a recent publication. We respond to this comment, and detail a calculation error in the original letter.
Münster, 2018
We discuss the Oscillating Resonant Group AxioN (ORGAN) experiment, a high mass axion haloscope h... more We discuss the Oscillating Resonant Group AxioN (ORGAN) experiment, a high mass axion haloscope hosted at the University of Western Australia (UWA). The path-finding experiment placed limits in a narrow frequency range around 26 GHz, and the future searches, which will be enabled by novel resonator designs based on dielectric structures, will focus on wider mass ranges with increased sensitivity. The next stage of the experiment will scan from 26-27 GHz, whilst the subsequent searches will focus on the 15-50 GHz range.
Physical Review Letters, 2021
The mass of axion dark matter is only weakly bounded by cosmological observations, necessitating ... more The mass of axion dark matter is only weakly bounded by cosmological observations, necessitating a variety of detection techniques over several orders of magnitude of mass ranges. Axions haloscopes based on resonant cavities have become the current standard to search for dark matter axions. Such structures are inherently narrowband and for low masses the volume of the required cavity becomes prohibitively large. Broadband low-mass detectors have already been proposed using inductive magnetometer sensors and a gapped toroidal solenoid magnet. In this work we propose an alternative, which uses electric sensors in a conventional solenoidal magnet aligned in the laboratory z-axis, as implemented in standard haloscope experiments. In the presence of the DC magnetic field, the inverse Primakoff effect causes a time varying permanent electric vacuum polarization in the z-direction to oscillate at the axion Compton frequency, which induces an oscillating electromotive force. We propose non-...
arXiv: Materials Science, 2015
The dielectric properties of strontium titanate (SrTiO$_3$) have previously been reported from ro... more The dielectric properties of strontium titanate (SrTiO$_3$) have previously been reported from room temperature to low temperatures with conflicting results. In this work, precision measurement of the permittivity is undertaken by simultaneously measuring transverse electric and transverse magnetic resonant modes within a single crystal. It is unequivocally shown that the permittivity is isotropic at room temperature with a permittivity of order 316.3pm2.2316.3\pm2.2316.3pm2.2 by measuring multiple modes of different electric field polarisations. As the crystal is cooled to 5 K and undergoes well known phase transitions, we show the material becomes uniaxial anisotropic with the ratio of the parallel to perpendicular permittivity to the cylinder z-axis of the sample as high as 2.4 below 6 K.
arXiv: Classical Physics, 2019
Consideration is given to the electrodynamics of bound charge and free charge alternating current... more Consideration is given to the electrodynamics of bound charge and free charge alternating current voltage sources. The bound charge problem is equivalent to the idealized bar electret, which is polarized uniformly parallel to its axis with a permanent impressed polarization of vecPbi(t)\vec{P}_b^i(t)vecPbi(t) and in general could be time dependent. For the free charge voltage source, a polarization vector may also be defined based on the separation of free charge, which is equivalent to the force per unit charge converted from the external energy which drives the voltage source. In this work we generalize the electrodynamic model of a bound charge electret voltage source and a free charge voltage source to a quasi-static time varying solution and apply the two-potential formulation. It is shown that an impressed effective magnetic current defines one of the boundary conditions of the voltage sources described by an electric vector potential. This comes about due to a generalization of Faraday's...
Physics of the Dark Universe, 2020
Physical Review Letters, 2021
Applied Physics Letters, 2020
A method for determining the internal DC magnetic field inside a superconducting cavity is presen... more A method for determining the internal DC magnetic field inside a superconducting cavity is presented. The method relies on the relationship between the magnetic field and frequency of the Kittel mode of a ferrimagnetic sphere, hybridized in the dispersive regime of the superconducting cavity. Results were used to experimentally determine the level of screening that a superconducting Nb cavity provides as it changes from perfect diamagnetism to no screening. Two cavity geometries were tested, a cylinder and single post re-entrant cavity. Both demonstrated a consistent value of field that enters the cavity, expected to be the superheating critical field. Hysteresis in the screened field during ramp up and ramp down of the external magnetic field due to trapped vortices was also observed. Some abnormal behavior was observed in the cylindrical cavity in the form of plateaus in the internal field above the first critical field, and we discuss the potential origin of this behavior. The me...
Applied Physics Letters, 2017
Electromagnetic properties of single crystal terbium gallium garnet are characterised from room d... more Electromagnetic properties of single crystal terbium gallium garnet are characterised from room down to millikelvin temperatures using the whispering gallery mode method. Microwave spectroscopy is performed at low powers equivalent to a few photons in energy and conducted as functions of the magnetic field and temperature. A phase transition is detected close to the temperature of 3.5 K. This is observed for multiple whispering gallery modes causing an abrupt negative frequency shift and a change in transmission due to extra losses in the new phase caused by a change in complex magnetic susceptibility.
Applied Physics Letters, 2016
Applied Physics Letters, 2014
Thermal Nyquist noise fluctuations of high-Q bulk acoustic wave cavities have been observed at cr... more Thermal Nyquist noise fluctuations of high-Q bulk acoustic wave cavities have been observed at cryogenic temperatures with a DC superconducting quantum interference device amplifier. High Q modes with bandwidths of few tens of milliHz produce thermal fluctuations with a signal-to-noise ratio of up to 23 dB. The estimated effective temperature from the Nyquist noise is in good agreement with the physical temperature of the device, confirming the validity of the equivalent circuit model and the non-existence of any excess resonator self-noise. The measurements also confirm that the quality factor remains extremely high (Q > 108 at low order overtones) for very weak (thermal) system motion at low temperatures, when compared to values measured with relatively strong external excitation. This result represents an enabling step towards operating such a high-Q acoustic device at the standard quantum limit.
2008 IEEE International Frequency Control Symposium, 2008
arXiv (Cornell University), Oct 18, 2017
Sensors
The creation of electromagnetic energy may be realised by engineering a device with a method of t... more The creation of electromagnetic energy may be realised by engineering a device with a method of transduction, which allows an external energy source, such as mechanical, chemical, nuclear, etc., to be impressed into the electromagnetic system through a mechanism that enables the separation of opposite polarity charges. For example, a voltage generator, such as a triboelectric nanogenerator, enables the separation of charges through the transduction of mechanical energy, creating an active physical dipole in the static case, or an active Hertzian dipole in the time-dependent case. The net result is the creation of a static or time-dependent permanent polarisation, respectively, without an applied electric field and with a non-zero vector curl. This system is the dual of a magnetic solenoid or permanent magnet excited by a circulating electrical current or fictitious bound current, respectively, which supplies a magnetomotive force described by a magnetic vector potential and a magnet...
The conversion of external energy into electricity is the foundation of power station and energy ... more The conversion of external energy into electricity is the foundation of power station and energy harvesting operation. The external source supplies an impressed force per unit charge to free or bound charge to produces AC electricity. We analyze the electrodynamics of ideal electricity generators through a time dependent permanent polarization without any applied electric field, which modifies the constitutive relations and is essential to oscillate charge in a lossless way. For both cases, we show that Maxwell's equations, and in particular Faraday's law are modified, along with the required boundary conditions through the addition of an effective impressed magnetic current boundary source. For the free charge case, we highlight the example of an electromagnetic generator based on Lorentz force, where the impressed force per unit charge that polarizes the conductor comes from mechanical motion of free charge with an impressed velocity of a conductor relative to a stationary...
A recent comment highlights a discussion at the PATRAS Workshop on Axions, WIMPs and WISPs relati... more A recent comment highlights a discussion at the PATRAS Workshop on Axions, WIMPs and WISPs relating to a recent publication. We respond to this comment, and detail a calculation error in the original letter.
Münster, 2018
We discuss the Oscillating Resonant Group AxioN (ORGAN) experiment, a high mass axion haloscope h... more We discuss the Oscillating Resonant Group AxioN (ORGAN) experiment, a high mass axion haloscope hosted at the University of Western Australia (UWA). The path-finding experiment placed limits in a narrow frequency range around 26 GHz, and the future searches, which will be enabled by novel resonator designs based on dielectric structures, will focus on wider mass ranges with increased sensitivity. The next stage of the experiment will scan from 26-27 GHz, whilst the subsequent searches will focus on the 15-50 GHz range.
Physical Review Letters, 2021
The mass of axion dark matter is only weakly bounded by cosmological observations, necessitating ... more The mass of axion dark matter is only weakly bounded by cosmological observations, necessitating a variety of detection techniques over several orders of magnitude of mass ranges. Axions haloscopes based on resonant cavities have become the current standard to search for dark matter axions. Such structures are inherently narrowband and for low masses the volume of the required cavity becomes prohibitively large. Broadband low-mass detectors have already been proposed using inductive magnetometer sensors and a gapped toroidal solenoid magnet. In this work we propose an alternative, which uses electric sensors in a conventional solenoidal magnet aligned in the laboratory z-axis, as implemented in standard haloscope experiments. In the presence of the DC magnetic field, the inverse Primakoff effect causes a time varying permanent electric vacuum polarization in the z-direction to oscillate at the axion Compton frequency, which induces an oscillating electromotive force. We propose non-...
arXiv: Materials Science, 2015
The dielectric properties of strontium titanate (SrTiO$_3$) have previously been reported from ro... more The dielectric properties of strontium titanate (SrTiO$_3$) have previously been reported from room temperature to low temperatures with conflicting results. In this work, precision measurement of the permittivity is undertaken by simultaneously measuring transverse electric and transverse magnetic resonant modes within a single crystal. It is unequivocally shown that the permittivity is isotropic at room temperature with a permittivity of order 316.3pm2.2316.3\pm2.2316.3pm2.2 by measuring multiple modes of different electric field polarisations. As the crystal is cooled to 5 K and undergoes well known phase transitions, we show the material becomes uniaxial anisotropic with the ratio of the parallel to perpendicular permittivity to the cylinder z-axis of the sample as high as 2.4 below 6 K.
arXiv: Classical Physics, 2019
Consideration is given to the electrodynamics of bound charge and free charge alternating current... more Consideration is given to the electrodynamics of bound charge and free charge alternating current voltage sources. The bound charge problem is equivalent to the idealized bar electret, which is polarized uniformly parallel to its axis with a permanent impressed polarization of vecPbi(t)\vec{P}_b^i(t)vecPbi(t) and in general could be time dependent. For the free charge voltage source, a polarization vector may also be defined based on the separation of free charge, which is equivalent to the force per unit charge converted from the external energy which drives the voltage source. In this work we generalize the electrodynamic model of a bound charge electret voltage source and a free charge voltage source to a quasi-static time varying solution and apply the two-potential formulation. It is shown that an impressed effective magnetic current defines one of the boundary conditions of the voltage sources described by an electric vector potential. This comes about due to a generalization of Faraday's...
Physics of the Dark Universe, 2020
Physical Review Letters, 2021
Applied Physics Letters, 2020
A method for determining the internal DC magnetic field inside a superconducting cavity is presen... more A method for determining the internal DC magnetic field inside a superconducting cavity is presented. The method relies on the relationship between the magnetic field and frequency of the Kittel mode of a ferrimagnetic sphere, hybridized in the dispersive regime of the superconducting cavity. Results were used to experimentally determine the level of screening that a superconducting Nb cavity provides as it changes from perfect diamagnetism to no screening. Two cavity geometries were tested, a cylinder and single post re-entrant cavity. Both demonstrated a consistent value of field that enters the cavity, expected to be the superheating critical field. Hysteresis in the screened field during ramp up and ramp down of the external magnetic field due to trapped vortices was also observed. Some abnormal behavior was observed in the cylindrical cavity in the form of plateaus in the internal field above the first critical field, and we discuss the potential origin of this behavior. The me...
Applied Physics Letters, 2017
Electromagnetic properties of single crystal terbium gallium garnet are characterised from room d... more Electromagnetic properties of single crystal terbium gallium garnet are characterised from room down to millikelvin temperatures using the whispering gallery mode method. Microwave spectroscopy is performed at low powers equivalent to a few photons in energy and conducted as functions of the magnetic field and temperature. A phase transition is detected close to the temperature of 3.5 K. This is observed for multiple whispering gallery modes causing an abrupt negative frequency shift and a change in transmission due to extra losses in the new phase caused by a change in complex magnetic susceptibility.
Applied Physics Letters, 2016
Applied Physics Letters, 2014
Thermal Nyquist noise fluctuations of high-Q bulk acoustic wave cavities have been observed at cr... more Thermal Nyquist noise fluctuations of high-Q bulk acoustic wave cavities have been observed at cryogenic temperatures with a DC superconducting quantum interference device amplifier. High Q modes with bandwidths of few tens of milliHz produce thermal fluctuations with a signal-to-noise ratio of up to 23 dB. The estimated effective temperature from the Nyquist noise is in good agreement with the physical temperature of the device, confirming the validity of the equivalent circuit model and the non-existence of any excess resonator self-noise. The measurements also confirm that the quality factor remains extremely high (Q > 108 at low order overtones) for very weak (thermal) system motion at low temperatures, when compared to values measured with relatively strong external excitation. This result represents an enabling step towards operating such a high-Q acoustic device at the standard quantum limit.
2008 IEEE International Frequency Control Symposium, 2008