Superfluid helium-4 is the superfluid form of helium-4, an isotope of the element helium. A superfluid is a state of matter in which matter behaves like a fluid with zero viscosity. The substance, which looks like a normal liquid, flows without friction past any surface, which allows it to continue to circulate over obstructions and through pores in containers which hold it, subject only to its own inertia.[1]

The formation of the superfluid is known to be related to the formation of a Bose–Einstein condensate. This is made obvious by the fact that superfluidity occurs in liquid helium-4 at far higher temperatures than it does in helium-3. Each atom of helium-4 is a boson particle, by virtue of its zero spin. Helium-3, however, is a fermion particle, which can form bosons only by pairing with itself at much lower temperatures, in a process similar to the electron pairing in superconductivity.[2]

History
Known as a major facet in the study of quantum hydrodynamics and macroscopic quantum phenomena, the superfluidity effect was discovered by Pyotr Kapitsa[3] and John F. Allen, and Don Misener[4] in 1937. Onnes possibly observed the superfluid phase transition on August 2 1911, the same day that he observed superconductivity in mercury.[5] It has since been described through phenomenological and microscopic theories.

In the 1950s, Hall and Vinen performed experiments establishing the existence of quantized vortex lines in superfluid helium.[6] In the 1960s, Rayfield and Reif established the existence of quantized vortex rings.[7] Packard has observed the intersection of vortex lines with the free surface of the fluid,[8] and Avenel and Varoquaux have studied the Josephson effect in superfluid helium-4.[9] In 2006, a group at the University of Maryland visualized quantized vortices by using small tracer particles of solid hydrogen.[10]

In the early 2000s, physicists created a Fermionic condensate from pairs of ultra-cold fermionic atoms. Under certain conditions, fermion pairs form diatomic molecules and undergo Bose–Einstein condensation. At the other limit, the fermions (most notably superconducting electrons) form Cooper pairs which also exhibit superfluidity. This work with ultra-cold atomic gases has allowed scientists to study the region in between these two extremes, known as the BEC-BCS crossover.

Supersolids may also have been discovered in 2004 by physicists at Penn State University. When helium-4 is cooled below about 200 mK under high pressures, a fraction (≈1%) of the solid appears to become superfluid.[11][12] By quench cooling or lengthening the annealing time, thus increasing or decreasing the defect density respectively, it was shown, via torsional oscillator experiment, that the supersolid fraction could be made to range from 20% to completely non-existent. This suggested that the supersolid nature of helium-4 is not intrinsic to helium-4 but a property of helium-4 and disorder.[13][14] Some emerging theories posit that the supersolid signal observed in helium-4 was actually an observation of either a superglass state[15] or intrinsically superfluid grain boundaries in the helium-4 crystal.[16]

Applications
Recently in the field of chemistry, superfluid helium-4 has been successfully used in spectroscopic techniques as a quantum solvent. Referred to as superfluid helium droplet spectroscopy (SHeDS), it is of great interest in studies of gas molecules, as a single molecule solvated in a superfluid medium allows a molecule to have effective rotational freedom, allowing it to behave similarly to how it would in the "gas" phase. Droplets of superfluid helium also have a characteristic temperature of about 0.4 K which cools the solvated molecule(s) to its ground or nearly ground rovibronic state.

Properties
See also: Helium § Helium II, and superfluidity
Superfluids, such as helium-4 below the lambda point, exhibit many unusual properties. A superfluid acts as if it were a mixture of a normal component, with all the properties of a normal fluid, and a superfluid component. The superfluid component has zero viscosity and zero entropy. Application of heat to a spot in superfluid helium results in a flow of the normal component which takes care of the heat transport at relatively high velocity (up to 20 cm/s) which leads to a very high effective thermal conductivity.

क्या आपने कभी सुना है कि हीलियम के उपर gravity का कोई affect नही होता! जी हां दोस्तो हीलियम हमेशा गुरुत्वाकर्षण से विपरीत दिशा में बहता है। जब इसे इसके boiling point, -269 डिग्री सेल्सियस या -452 डिग्री फ़ारेनहाइट से कुछ डिग्री नीचे ठंडा किया जाता है, तो यह एक सुपरफ्लुइड में बदल जाता है। और superfluid बनने के बाद यह घर्षणहीन यानि frictionless हो जाता है, और कंटेनर के ऊपर की ओर उल्टा बहने लगते है यहां तक कि कंटेनरों में मौजूद छोटी-छोटी दरारों के माध्यम से भी ये बाहर निकल सकता हैं. ऐसे ही वैज्ञानिक रहस्यों से भरी सारी जानकारी को जानने के लिए आये साइंस को सब्सक्राइब करें।

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