Implement jQuery AJAX call with Entity Framework in ASP.Net

I am working entity frame work .i have built a function which which check user already exist or not  data base so that I that problem I am Jason with a web method but it is not work following my  code.

JQuey Code
     <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js" type="text/javascript"> </script>  
   <script type="text/javascript">

       function UserAvailability() { //This function call on text change.             
           $.ajax({
               type: "POST",
               url: "AddUser.aspx/CheckUser", // this for calling the web method function in cs code.  Check user is my webmethod.
               data: '{username: "' + $("#<%=txtUserName.ClientID%>")[0].value + '" }', // user name   
               contentType: "application/json; charset=utf-8",
               dataType: "json",
               success: OnSuccess,
               failure: function (response) {
                   alert(response);
               }
           });
       }

       // function OnSuccess  
       function OnSuccess(response) {
           var msg = $("#<%=lblStatus.ClientID%>")[0];
           switch (response.d) {
               case "true":
                   msg.style.display = "block";
                   msg.style.color = "red";
                   msg.innerHTML = "User Name already exists.";
                   break;
               case "false":
                   msg.style.display = "block";
                   msg.style.color = "green";
                   msg.innerHTML = "User Name Available";
                   break;
           }
       }  
   </script>

///////Text Box where call    /////////////////
     <asp:TextBox ID="TextBoxUserName" autofocus required placeholder="User Name" runat="server" onchange="UserAvailability()"></asp:TextBox>
                                      
                            </div>
                            <div id="checkusernameoremail" runat="server">                             
                            <asp:Label ID="lblStatus" runat="server" ></asp:Label>  
                        </div> 

/////////////////////////

My stored procedure…
ALTER PROCEDURE User_CheckUserAvailablity
      @UserName varchar(50)
AS
BEGIN
      SET NOCOUNT ON;
      IF NOT EXISTS
            (SELECT * FROM Users
             WHERE UserName = @UserName
            )
            SELECT 'true'
      ELSE
            SELECT 'false'
END

Below my web method …
[System.Web.Services.WebMethod]
    public static string CheckUser(User username) -------User is my Table Name and username is object.
        {
            string retval = "";
            NewEntities db = new NewEntities();--------NewEntites are my EDMX Name
            db.User_CheckUserAvailablity(------User_Check Availability  are my stored procedure name
              username.UserName
                );
        return retval;
        }  

I think problem in my web method  please help me to solve my problem.