Physicists at the California Institute of Technology have announced the first observation of the quantum of thermal conductance. This discovery reveals a fundamental limit to the heat that can be conducted by objects of atomic dimensions.
The findings, reported in the April 27 issue of the journal Nature, could have profound implications for the future design of microscopic electronic devices and for the transmission of information, according to the research team leader, Caltech physics professor Michael Roukes.
The quantum of thermal conductance is best understood by beginning with a simple explanation of heat flow. In the everyday world, the amount of heat carried by an object can vary in a smooth and continuous way. Heat actually flows by means of collective, wavelike vibrations of the atoms that make up a solid material. Usually immense numbers of such waves, each inducing a unique type of synchronous motion of the atoms, act simultaneously to carry heat along a material.
Physicists know that waves sometimes act like particles and vice versa, so they've given these vibrations the particle-like name phonon (reminiscent of "electron" but named after the Greek root phon for sound.) For heat flow in the macroworld, since each phonon is just one among a sea of many others, an individual phonon's contribution alters the total only imperceptibly.
But in the nanoworld, this "phonon sea" is actually rather finite, quantum effects rule, and the heat conduction can become radically different. When an object becomes extremely small, only a limited number of phonons remain active and play a significant role in heat flow within it. In fact, in small devices at temperatures close to absolute zero, most types of motion become almost completely "frozen out," and heat must then be carried by only the several remaining types of wavelike motions that persist.
It has recently become apparent that, in this regime, a strict limit exists to the amount of heat that can be conducted in a small structure or device. Although never before observed, this maximum value is actually a fundamental law of nature, independent of composition or material. It stipulates that the only way thermal conductance can be increased in a very small device is simply to make the conductor larger.
The findings, reported in the April 27 issue of the journal Nature, could have profound implications for the future design of microscopic electronic devices and for the transmission of information, according to the research team leader, Caltech physics professor Michael Roukes.
The quantum of thermal conductance is best understood by beginning with a simple explanation of heat flow. In the everyday world, the amount of heat carried by an object can vary in a smooth and continuous way. Heat actually flows by means of collective, wavelike vibrations of the atoms that make up a solid material. Usually immense numbers of such waves, each inducing a unique type of synchronous motion of the atoms, act simultaneously to carry heat along a material.
Physicists know that waves sometimes act like particles and vice versa, so they've given these vibrations the particle-like name phonon (reminiscent of "electron" but named after the Greek root phon for sound.) For heat flow in the macroworld, since each phonon is just one among a sea of many others, an individual phonon's contribution alters the total only imperceptibly.
But in the nanoworld, this "phonon sea" is actually rather finite, quantum effects rule, and the heat conduction can become radically different. When an object becomes extremely small, only a limited number of phonons remain active and play a significant role in heat flow within it. In fact, in small devices at temperatures close to absolute zero, most types of motion become almost completely "frozen out," and heat must then be carried by only the several remaining types of wavelike motions that persist.
It has recently become apparent that, in this regime, a strict limit exists to the amount of heat that can be conducted in a small structure or device. Although never before observed, this maximum value is actually a fundamental law of nature, independent of composition or material. It stipulates that the only way thermal conductance can be increased in a very small device is simply to make the conductor larger.
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