The introduction of carbon interstitials into high-entropy alloys(HEAs)provides an effective way to improve their properties.However,all such HEA systems explored so far are limited to those with the face-centered-cub...The introduction of carbon interstitials into high-entropy alloys(HEAs)provides an effective way to improve their properties.However,all such HEA systems explored so far are limited to those with the face-centered-cubic(fcc)structure.Here we report the structural,mechanical and physical properties of the refractory(Nb_(0.375)Ta_(0.25)Mo_(0.125)W_(0.125)Re_(0.125))_(100−x)C_(x) HEAs over a wide x range of 0≤x≤20.It is found that,whereas the starting HEA(x=0)is composed of a major body-centered-cubic(bcc)phase with significant impurities,the bcc phase fraction increases with the C concentration and achieves almost 100%at x=20.Moreover,the increase of C content x results in an expansion of the bcc lattice,an enhancement of the microhardness,an increase in residual resistivity and a small variation of density of states at the Fermi level.All these features are consistent with the expectation that carbon atoms occupy the interstitial site.For x≥11.1,the X-ray photoelectron spectroscopy indicates the bond formation between the carbon and metal atoms,suggesting that some carbon atoms may also reside in the lattice site.In addition,a semiquantitative analysis shows that the enhanced mixing entropy caused by carbon addition plays a key role in stabilizing the(nearly)single solid-solution phase.Our study not only provides the first series of carbon interstitial HEAs with a bcc structure,but also helps to better understand the alloying behavior of carbon in refractory HEAs.展开更多
High-entropy alloys(HEAs)are the focus of current research for their diverse properties,including superconductivity and structural polymorphism.However,the polymorphic transition has been observed only in nonsupercond...High-entropy alloys(HEAs)are the focus of current research for their diverse properties,including superconductivity and structural polymorphism.However,the polymorphic transition has been observed only in nonsuperconducting HEAs mostly under high pressure.Here we report the discovery of the superconductivity and temperature-driven polymorphism in(V0.5Nb0.5)3-xMoxAl0.5Ga0.5(0.2≤x≤1.4)HEAs,which are of a single body-centered cubic(bcc)structure for x=0.2 and a mixture of the bcc and A15 structures for higher x values.Upon annealing,the bcc structure undergoes a polymorphic transformation to the A15 one and all HEAs exhibit bulk superconductivity.For the sample with x=0.2,the bcc polymorph is not superconducting down to1.8 K,whereas the A15 polymorph has a superconducting transition temperature Tc of 10.2 K and estimated zero-temperature upper critical field Bc2(0)of 20.1 T,both of which are the highest among HEA superconductors.With increasing Mo content x,both Tc and Bc2(0)of the A15-type HEAs decrease,yet the large ratio of Bc2(0)/Tc signifies a disorder-induced enhancement of the upper critical field over a wide x range.The decrease in Tc is attributed to the decrease in both the electronic specific-heat coefficient and electron-phonon coupling strength.Furthermore,the valence electron count dependence of Tc,which is different from both the binary A15 and other structurally different HEA superconductors,suggests that Tc may be increased further by reducing the number of valence electrons.Our results not only uncover HEA superconductors of a new structural type,but also provide the first example of polymorphism-dependent superconductivity in HEAs.展开更多
A new noncentrosymmetric phosphide ThIrP has been synthesized and characterized. X-ray difraction analysis shows that this compound crystallizes in a LaPtSi-type tetragonal lattice(space group I41md, Z = 4), whose lat...A new noncentrosymmetric phosphide ThIrP has been synthesized and characterized. X-ray difraction analysis shows that this compound crystallizes in a LaPtSi-type tetragonal lattice(space group I41md, Z = 4), whose lattice parameters are a = b =4.0676(1) ?, c = 14.3354(2) ?, and V = 237.191(8) ?3. Moreover, ThIrP is discovered to be an intermediately coupled,type-Ⅱ superconductor with possibly multiple gaps below Tc= 5.07 K. The upper critical magnetic field, Sommerfield coefficient, and Ginzburg-Laudau parameter are determined based on physical property measurements, which are Bc2= 0.83 T, γ =7.5 mJ mol-1 K-2, and κGL= 7.5, respectively. The electronic band structure calculations point out nearly equal contributions of Ir and Th atoms on the density of states around the Fermi surface. In addition, the spin-orbit coupling induced band splitting reaches as large as 270 meV along the Γ-Z line. Our results suggest that ThIrP provides a platform to study the interplay between inversion-symmetry breaking, strong spin-orbit coupling, and superconductivity.展开更多
In this work,we systematically studied the magnetic and transport properties of EuAg4As2 single crystals.It was found that the two antiferromagnetic transitions(TN1=10 K and TN2=15 K)were driven to lower temperatures ...In this work,we systematically studied the magnetic and transport properties of EuAg4As2 single crystals.It was found that the two antiferromagnetic transitions(TN1=10 K and TN2=15 K)were driven to lower temperatures by an applied magnetic field.Below TN1,two successive metamagnetic transitions were observed when a magnetic field was applied in the ab plane(H//abplane).For both H//ab and H//c,EuAg4As2 showed a positive,unexpectedly large magnetoresistance(up to 202%)in lower magnetic fields below TN1,and a large negative magnetoresistance(up to-78%)at high fields/intermediate temperatures,thus presenting potential applications in magnetic sensors.Finally,the magnetic phase diagrams of EuAg4As2 were constructed for both H//ab and H//c using the resistivity and magnetisation data.展开更多
基金financial support by the foundation of Westlake University and the Service Center for Physical Sciences for technical assistance in SEM measurementssupported by the National Natural Science Foundation of China(12050003)。
基金the foundation of Westlake University for financial supportThe work at Zhejiang University was supported by the National Key Research and Development Program of China(2017YFA0303002)。
文摘The introduction of carbon interstitials into high-entropy alloys(HEAs)provides an effective way to improve their properties.However,all such HEA systems explored so far are limited to those with the face-centered-cubic(fcc)structure.Here we report the structural,mechanical and physical properties of the refractory(Nb_(0.375)Ta_(0.25)Mo_(0.125)W_(0.125)Re_(0.125))_(100−x)C_(x) HEAs over a wide x range of 0≤x≤20.It is found that,whereas the starting HEA(x=0)is composed of a major body-centered-cubic(bcc)phase with significant impurities,the bcc phase fraction increases with the C concentration and achieves almost 100%at x=20.Moreover,the increase of C content x results in an expansion of the bcc lattice,an enhancement of the microhardness,an increase in residual resistivity and a small variation of density of states at the Fermi level.All these features are consistent with the expectation that carbon atoms occupy the interstitial site.For x≥11.1,the X-ray photoelectron spectroscopy indicates the bond formation between the carbon and metal atoms,suggesting that some carbon atoms may also reside in the lattice site.In addition,a semiquantitative analysis shows that the enhanced mixing entropy caused by carbon addition plays a key role in stabilizing the(nearly)single solid-solution phase.Our study not only provides the first series of carbon interstitial HEAs with a bcc structure,but also helps to better understand the alloying behavior of carbon in refractory HEAs.
基金supported by the National Key Research&Development Program of China(2017YFA0303002).
文摘High-entropy alloys(HEAs)are the focus of current research for their diverse properties,including superconductivity and structural polymorphism.However,the polymorphic transition has been observed only in nonsuperconducting HEAs mostly under high pressure.Here we report the discovery of the superconductivity and temperature-driven polymorphism in(V0.5Nb0.5)3-xMoxAl0.5Ga0.5(0.2≤x≤1.4)HEAs,which are of a single body-centered cubic(bcc)structure for x=0.2 and a mixture of the bcc and A15 structures for higher x values.Upon annealing,the bcc structure undergoes a polymorphic transformation to the A15 one and all HEAs exhibit bulk superconductivity.For the sample with x=0.2,the bcc polymorph is not superconducting down to1.8 K,whereas the A15 polymorph has a superconducting transition temperature Tc of 10.2 K and estimated zero-temperature upper critical field Bc2(0)of 20.1 T,both of which are the highest among HEA superconductors.With increasing Mo content x,both Tc and Bc2(0)of the A15-type HEAs decrease,yet the large ratio of Bc2(0)/Tc signifies a disorder-induced enhancement of the upper critical field over a wide x range.The decrease in Tc is attributed to the decrease in both the electronic specific-heat coefficient and electron-phonon coupling strength.Furthermore,the valence electron count dependence of Tc,which is different from both the binary A15 and other structurally different HEA superconductors,suggests that Tc may be increased further by reducing the number of valence electrons.Our results not only uncover HEA superconductors of a new structural type,but also provide the first example of polymorphism-dependent superconductivity in HEAs.
基金financial support by the foundation of Westlake Universitysupported by the National Key Research Development Program of China (Grant No. 2017YFA0303002)the Fundamental Research Funds for the Central Universities of China。
文摘A new noncentrosymmetric phosphide ThIrP has been synthesized and characterized. X-ray difraction analysis shows that this compound crystallizes in a LaPtSi-type tetragonal lattice(space group I41md, Z = 4), whose lattice parameters are a = b =4.0676(1) ?, c = 14.3354(2) ?, and V = 237.191(8) ?3. Moreover, ThIrP is discovered to be an intermediately coupled,type-Ⅱ superconductor with possibly multiple gaps below Tc= 5.07 K. The upper critical magnetic field, Sommerfield coefficient, and Ginzburg-Laudau parameter are determined based on physical property measurements, which are Bc2= 0.83 T, γ =7.5 mJ mol-1 K-2, and κGL= 7.5, respectively. The electronic band structure calculations point out nearly equal contributions of Ir and Th atoms on the density of states around the Fermi surface. In addition, the spin-orbit coupling induced band splitting reaches as large as 270 meV along the Γ-Z line. Our results suggest that ThIrP provides a platform to study the interplay between inversion-symmetry breaking, strong spin-orbit coupling, and superconductivity.
基金supported by the Ministry of Science and Technology of China(Grant No.2016YFA0300402)the National Natural Science Foundation of China(Grant Nos.11974095,and 12074335)+1 种基金the Zhejiang Natural Science Foundation(Grant No.LY16A040012)the Fundamental Research Funds for the Central Universities。
文摘In this work,we systematically studied the magnetic and transport properties of EuAg4As2 single crystals.It was found that the two antiferromagnetic transitions(TN1=10 K and TN2=15 K)were driven to lower temperatures by an applied magnetic field.Below TN1,two successive metamagnetic transitions were observed when a magnetic field was applied in the ab plane(H//abplane).For both H//ab and H//c,EuAg4As2 showed a positive,unexpectedly large magnetoresistance(up to 202%)in lower magnetic fields below TN1,and a large negative magnetoresistance(up to-78%)at high fields/intermediate temperatures,thus presenting potential applications in magnetic sensors.Finally,the magnetic phase diagrams of EuAg4As2 were constructed for both H//ab and H//c using the resistivity and magnetisation data.
